(Background: It can be shown that the statement, "Whoever lives outside of Eretz Yisroel is as if he worships idols" (Kesubos 110b) does not apply nowadays, because the reason is that when one lives under a government of people who serve idols, he is indirectly serving the idols. This is only when one leaves a Jewish kingdom of believers in Hashem, but in exile when even Eretz Yisroel is not under Jewish control, one is an indirect idol worshipper wherever he goes.)
Further proof that the statement "Whoever lives outside of Eretz Yisroel is as if he worships idols" does not apply nowadays can be found in the fact that the Rif, the Rosh, the Tur and the Shulchan Aruch do not incorporate this statement into their codes, although they bring all the detailed laws of husband and wife forcing one another to move to Eretz Yisroel. These authorities only codify halacha that applies nowadays, and that is why they omitted this statement.
One might try to refute this proof by saying that the reason they omitted it was because they felt that it was already included and implied in the law that one spouse can force the other to move to Eretz Yisroel. (The basis of this argument is that Tosafos quotes Rabbeinu Chaim who says that there is no mitzvah to live in Eretz Yisroel nowadays, and therefore the law the one spouse can force the other does not apply. The implication of Tosafos is that whoever disagrees with Rabbeinu Chaim and holds that the law of forcing does apply, would also hold that there is a mitzvah to live in Eretz Yisroel nowadays.)
However, this is not a true logical argument, since we see that the Ritva in Yoma 38 (quoted in Siman 4) holds that now, when we are under a decree of exile, there is no mitzvah or obligation to go to Eretz Yisroel, only that one who lives there is forbidden to leave. Yet the poskim did not consider this a proof that the Ritva held like Rabbeinu Chaim that that law of forcing does not apply today, otherwise they would have grouped the Ritva together with Rabbeinu Chaim, and in fact we find that the poskim (see Me'il Tzedaka, in the sources for Siman 3, page 3) say that Rabbeinu Chaim was a lone opinion. If the Ritva had disagreed with the Rif and Rabbeinu Meir and the other Rishonim who hold that the law of forcing does apply, the poskim would have said so, as they usually do when there are divergent opinions. So we must say that there is no proof from the law of forcing that there is an obligatory mitzvah to go to Eretz Yisroel.
Furthermore, the Mishnah says that one spouse can force the other to move to Jerusalem, and we do not find anywhere that there is an obligatory mitzvah to move from other parts of Eretz Yisroel to Jerusalem. The mitzvah of living in Eretz Yisroel, as the Ramban sets it down, applies equally to all parts of Eretz Yisroel. And no one would dream of saying that that someone leaving Jerusalem and moving to another part of Eretz Yisroel is as if he worshipped idols. Yet there is a law that one spouse can force the other to move to Jerusalem. The Chasan Sofer, Yoreh Deah 233 and 234 writes, based on the law of forcing, that since Jerusalem has more holiness than the rest of Eretz Yisroel, one who lives there is fulfilling a greater mitzvah. And he works hard to find excuses as to why most great rabbis for many generations lived in Tzfas rather than Jerusalem. So we must conclude that although it is not an obligatory mitzvah, still, since the place is holier, one's service to G-d there is greater, and therefore there is a slight mitzvah to live there and one spouse may force the other.
Of course, in the times of the Temple there was an even greater mitzvah to live in Jerusalem, since there were many mitzvos that could only be done there, like eating sacrifices and the second tithe and prayer in the Temple. Rabbi Aharon Halevi in the Sefer Hachinuch says that the reason why the Torah commanded us to bring the second tithe to Jerusalem and eat it there is so that by spending time in Jerusalem, one would see the service of the Kohanim in the Temple and become closer to the service of Hashem by witnessing all these actions. However, today we have unfortunately lost all of these things; still one spouse can force the other to move to Jerusalem, and it must be as we explained it above.
There are actually some poskim who bring proof that there is a mitzvah to live in Eretz Yisroel even nowadays from the fact that the halacha codes include the laws of one spouse forcing the other to go to Eretz Yisroel. But they only mean to bring proof that we don't rule in accordance with Rabbeinu Chaim, who says that the law of forcing doesn't apply nowadays since there is no mitzvah to live in Eretz Yisroel, no mitzvah at all - on this these poskim write that since the other Rishonim hold that the law of forcing does apply even today, it must be that they disagree with Rabbeinu Chaim, and they hold that there is at least some mitzvah even today.
But those later poskim (like the Me'il Tzedaka and the Maharit) who hold that there is a real Torah mitzvah to live in Eretz Yisroel even today are basing themselves not on the law of forcing, but on the Ramban and the Sifri quoted by the Ramban. This is not the place to write at length on this subject, but in any case their proof is not from the law of forcing.
(See, however, the Ramban in his commentary on Bamidbar 33:53 where he does cite the law of forcing one's spouse as proof that there is a mitzvah to live in Eretz Yisroel even nowadays. How then would the Ramban deal with the Rebbe's point that there is no mitzvah to live in Jerusalem, yet one can force one's spouse to move there? Doesn't this show that the right to force in no way implies that it's a mitzvah? Perhaps we could say that the Ramban held that Jerusalem towers above the rest of Eretz Yisroel in holiness more than Eretz Yisroel towers above the rest of the world. Therefore, we understand the idea of forcing to move to Jerusalem without a mitzvah, but the idea of forcing to move to Eretz Yisroel is impossible to understand, says the Ramban, unless we say there is a mitzvah.)
Therefore, one cannot argue that the Rif and the Rosh and the rest of the Rishonim who omit the statement that one who lives outside of Eretz Yisroel is as if he worshipped idols were relying on the fact that they codify the law of forcing, because this would not prove that it is such a big obligation that anyone who stays outside Eretz Yisroel would be like an idol worshipper. Rather, it must be that they held that the statement about worshipping idols does not apply nowadays, as I have proven earlier from the words of the Targumim, which were written based on the teachings of the early Tannaim, and also from Rashi and other Rishonim.
However, the Rambam (Hilchos Melachim 5:12) does incorporate the statement about idol worship, although it does not apply today, because the Rambam's code includes all laws, even those that do not apply today.
According to all of the above (Simanim 4-6), we can understand why Rabbeinu Meir of Rothenberg says that the laws of forcing a spouse do not completely apply today. The reason is because in Temple times living in Eretz Yisroel was a great mitzvah and a Torah obligation, and one who left Eretz Yisroel was as if he worshipped idols, whereas nowadays both of these aspects are weaker: The idea that it is as if he worshipped idols no longer applies, and the mitzvah is no longer an obligation, due to the decree of exile as the Ritva says. There is still somewhat of a mitzvah, just as living in Jerusalem is a higher level than living in other parts of Eretz Yisroel, but this is definitely considered a small mitzvah relative to the mitzvah that existed in Temple times. And that is why the law of forcing a spouse was downgraded today.
It remains to be understood, however, why this meant a weakening of the wife's power and not the husband's. This is what the Tur asks, and the Beis Yosef's answer - that they still did not want to reduce the power of the husband - is not clear, since he gives no reason. I have already quoted the Bach's distinction between husband and wife (Siman 3) but now we are trying to explain the weakening of the wife's power in terms of the weakening of the mitzvah, whereas the Bach's concept applies even if the mitzvah now is equal to the mitzvah then.
(See Igros Moshe, on the last page of the first sources file, who proposes that the small optional mitzvah of living in Eretz Yisroel nowadays was enough to elevate Eretz Yisroel to the level of another city in the same country, where the husband has the exclusive right to force his wife to move.)