(Background: Rabbi Meir of Rothenberg stated that nowadays, only a husband can force a wife to immigrate to Eretz Yisroel, but a wife cannot force a husband, unlike in Temple times when both spouses had this power. The Rebbe has explained in simanim 4-6 that there is still a mitzvah nowadays, but it is weaker. The question remains: why does this weakness affect the wife more than the husband?)
I would propose that the reason why the wife's power was downgraded nowadays, but not the husband's, is because we suspect her of wanting to move to Eretz Yisroel for ulterior motives. We find this concept in Shulchan Aruch, Even Hoezer 75:4. There the Rema says that if a husband demanded that his wife move to Eretz Yisroel with him, she refused, and accordingly the law exempted him from paying the kesubah, and then he subsequently returned from Eretz Yisroel, even after many years - he must then pay the kesubah to her or her heirs. The Beis Shmuel explains that this is because his move to Eretz Yisroel has now been exposed as nothing more than a trick to avoid paying the kesubah.
Now, by Torah law a husband can divorce his wife whenever he wants, even against her will, so he doesn't need to play tricks to escape from the marriage. He needs the trick only to avoid paying the kesubah. A wife, by contrast, does not need any trick to get the kesubah; she is automatically entitled to it. She does need a trick if she wants to end the marriage and her husband refuses to grant the divorce.
In fact, the Mishnah in Nedarim 90b gives an example of a wife playing such a trick. The Mishnah states that originally, the law was that a kohein's wife who said “I was defiled by another man against my will” is believed, and her husband must divorce her and pay the kesubah. (This situation is unique to a kohein because an ordinary Jew may stay married to his wife if she was raped. And if she willfully committed adultery, he divorces her without paying the kesubah. Only a kohein is forbidden to stay married to his wife after she was raped, but since it was not her fault, she does not lose the kesubah.) But a later generation of Sages changed this law, out of fear that the wife might be making up the whole story, in order to escape from the marriage and marry another man whom she finds more attractive.
The Ran (Rabbeinu Nissim ben Reuven, 14th century Spain) asks: if originally the wife was believed, and she was thus forbidden under Torah law to stay married to him, how could the later Sages have permitted her because she might be playing a trick? The Ran quotes others who answered that the Sages have the power to occasionally suspend Torah law if the situation requires it. The Ran disagrees, arguing that the Sages only have the power to prohibit us from performing a positive commandment (e.g. blowing shofar on Shabbos), but not to tell us to transgress a negative commandment, such as the prohibition to continue a marriage after the wife was defiled. Only occasionally may they suspend a negative commandment, such as Eliyahu the prophet did when he built an altar on Mount Carmel, but not permanently. The Ran's final answer is that according to Torah law a wife does not have the right to claim that she is defiled and thus break up her marriage. The original law of the Mishnah, then, was actually only a Rabbinic stringency, and so when the Rabbis saw fit they annulled it.
If so, we must ask: why does the Mishnah give a wife the right to force her husband to move to Eretz Yisroel or else divorce her? Perhaps she is making this demand only because she wants to escape from the marriage and marry another man. Now, normally we would not ask such a question, because we can't compare one Rabbinic decree to another. Wherever the Sages decided to make a decree, they made one, not elsewhere. Possibly they did not wish to suspect her of trickery in the case of Eretz Yisroel, because she may be suffering physically in her current place of residence. Possibly the Sages did not want to prevent her from doing the mitzvah of moving to Eretz Yisroel. Or perhaps the Sages distinguished between the claim of defilement, which requires no effort on her part, just words, and the demand to move to Eretz Yisroel, where we grant her the divorce and kesubah only if she actually goes to the trouble of moving to Eretz Yisroel.
However, the question goes deeper, because we do actually find that the Sages worried about someone using emigration to Eretz Yisroel as a trick. The slave who runs away to Eretz Yisroel must be freed by his master under Torah law, yet the Sages enacted that such a slave must write his master a promissory note for his own value. Tosafos (Kesubos 110b) explains that the Sages were afraid that any slave wanting freedom would simply run to Eretz Yisroel, so they enacted that slaves pay a hefty price, to discourage all but the most ardent lovers of Eretz Yisroel. So we see that although slaves wanting to be free is not such a bad thing, and on the contrary a freed slave becomes a full-fledged Jew, obligated in all mitzvos, still the Sages were worried about the loss of masters, should all slaves run away. If so, they should have been even more concerned about wives looking at other men and attempting to break apart their marriages - a problem so great that it prompted the Sages to permit a Torah prohibition, at least according to the first opinion quoted by the Ran. As the Talmud says (Chullin 11b), there is no guard good enough to prevent people from transgressing marital prohibitions. If we leave the door open for wives to force divorce, who knows what may come next?
Still, we can't compare one Rabbinic law to another, as the Mishnah states (Yadayim 3:2), even when the logic of kal vachomer (a fortiori) applies, as the Eliyah Rabbah points out in his commentary there. So we cannot ask this as a question. However, we can use this idea to explain the Yerushalmi quoted by Rabbi Meir of Rothenberg, which says that in fact wives nowadays do not have the right to demand that their husbands move to Eretz Yisroel. In the times of the Temple, when moving to Eretz Yisroel was a Torah obligation, and living outside Eretz Yisroel was like worshipping idols, the Sages gave the wife the power to force her husband to move or divorce her, and they did not suspect her of having the ulterior motive of marry someone else. But nowadays, the slight mitzvah that still exists in moving to Eretz Yisroel was outweighed by the consideration of preventing wives from using this as a device to escape from the marriage.
One might ask: if so, the Sages should also have been concerned about the husband using the same device to escape from the marriage, because perhaps he found another woman he wants to marry. Now, in Talmudic times this was not a problem at all, because the Torah allows a man to divorce his wife even against her will, so there is no need for him to move to Eretz Yisroel to accomplish this. (One may ask why indeed the Torah permits this - doesn't it leave the door open for promiscuity on the man's part? The answer is that the Torah also allows a man to marry a second wife without divorcing the first, so if he does divorce her, it will not be the desire for another woman that motivates him.) But one might still contend that after the decree of Rabbeinu Gershom (10th century German teacher of Torah) that one may not divorce one's wife against her will, the law should change, and we should no longer allow a husband to force his wife to come with him to Eretz Yisroel or else accept a divorce, lest he use this as a vehicle to circumvent the Rabbi Gershom's decree. There are indeed opinions in Even Hoezer 178:9 that in the post-Rabbeinu Gershom era, we do not allow a man to claim that he believes a witness who says that his wife was defiled, because he might be using this as a trick to divorce her against her will and marry someone else.
But the answer is that the slight mitzvah that exists in moving to Eretz Yisroel outweighs the concern about him circumventing Rabbeinu Gershom, and if we are to be concerned about Rabbeinu Gershom, the entire enactment of the Talmudic Sages that one spouse can force the other to move to Eretz Yisroel would, in effect, become null and void. Rabbeinu Gershom did not make his decree so strong as to outweigh a mitzvah. Of course, if he is merely using moving to Eretz Yisroel as a device, it is not a mitzvah, but the point is that if we are going to suspect every husband of using it as a device, the end result will be that no husband will benefit from the enactment of the Talmudic Sages, even one who wants to move to Eretz Yisroel for the right reasons, since we cannot read minds. So why should we annul a Talmudic law in favor of Rabbeinu Gershom's law?
Besides, the law of the husband forcing his wife was enacted in Talmudic times, so we cannot change it so easily, even if circumstances have now changed (i.e. Rabbeinu Gershom's decree is in force). And this is especially so, since there is good reason for keeping the Talmudic law in force now as well (going to Eretz Yisroel outweighs Rabbeinu Gershom). So there is no comparison between the husband and the wife, who is forbidden by the Torah from marrying someone else, and we certainly do not want her setting her eyes on another man and forcing her husband to divorce her. So it is easy to understand why the Yerushalmi says that the husband has the power to force and the wife does not.
According to the above, we can also understand why there is a difference between a wife and a slave - the slave can force his master to free him even nowadays, while the wife cannot force her husband to divorce her nowadays. First of all, in the case of the slave it was possible to deter those with ulterior motives by making them write a promissory note to pay their masters for their freedom, while in the case of the wife no such monetary penalty would help - if she has set her eyes upon marrying someone else, no penalty would stop her. "There is no guard good enough to prevent people from transgressing marital prohibitions." Secondly, the danger of wives initiating divorce and marrying someone else is a more serious problem than slaves going free, which is merely a matter of financial loss for the masters. In general, divorce is worse than freeing a slave. We see that husband and wife living together is such an important matter that the Gemara at the end of Chullin (141a) says that we need a special verse in the Torah to teach that one may not violate the commandment against taking a mother bird sitting on her young even if one needs that bird to purify someone afflicted with tzoraas, and thus permit him to live with his wife. Although both of these (the obligation to send away the mother bird, and the obligation of the person with tzoraas to perform the ritual with the bird (Vayikra 14)) are positive commandments, and seemingly one should not be stronger than the other, still one would have thought that peace between husband and wife outweighs another commandment, since we find that Hashem allows His name to be erased (in the Sotah ritual) in order to restore peace between husband and wife.
Furthermore, freeing a slave who runs away to Eretz Yisroel is an explicit verse in the Torah (Devarim 23:16), and so it is clear that the Torah doesn't require us to suspect the slave of running to Eretz Yisroel just to become free. The Torah was given to humans who cannot read minds. The law about divorcing a wife who wants to go to Eretz Yisroel, on the other hand, is Rabbinic in origin, and thus the Sages might well have suspended in cases when the mitzvah to move is not so great and we don't know her true motives.