I'm not really religious anymore, but I still have an interest in Ancient Near Eastern religion, specifically that of the Hebrew/Jews, and how it influenced the emergence of Christianity in the first century.
In as much as I can tell, the tribes that would later makeup the nation-state of Israel (and later Israel and Judah) practiced polygamy, much like their Ancient Near Eastern / Mesopotamian neighbors. Robert Alter suggests this was partially because of the belief in semen as a limited commodity and the proliferation and preservation of a given family line (in other words, a patriarch having as many kids as possible, but making sure they are his kids). I'd welcome thoughts on this.
However, it seems that after a rapid succession of occupiers (Assyria/Babylon/Persia) and the diaspora, Judaism became mostly monogamous. What changed that made monogamy more pragmatic? When did this occur? What were the cultural incentives?
Please keep answers objective. I am not looking for a spiritual explanation or support for anti-Semitism.
I figure some answers may not be safe for work (such as the above explanation regarding semen), so marking this NSFW to be safe.
Concerning Rabbeinu Gershom's polygamy ban, I'd just like to add a little something to what u/hannahstohelit said in the post they linked!
In his book Pious and Rebellious: Jewish Women in Medieval Europe, Avraham Grossman argues that there were some particular aspects of life for Ashkenazi women that made polygamy a big problem in their communities. Many Ashkenazi Jewish men had itinerant careers, particularly as merchants (since other trades were sometimes closed to them). It was a chronic problem for men to stay away for many years and remarry somewhere else, abandoning their first family. This was technically allowed because of the lack of rulings against polygamy, and sometimes it was due to difficulties in returning home which could affect Jewish men who were particularly vulnerable as members of an often persecuted religious minority. But it caused a disaster for the wife back home who was very limited in her economic options to support her family without a husband. Sometimes women in this position were able to petition for some sort of economic resolution, but often this was not possible.
Outlawing polygamy was therefore a way to protect the interests of women whose husbands spent long periods of time away from home, since it made them unable to abandon their wives and start a new family abroad. I'm not really familiar with pre-Ashkenazi Jewish history, but it's possible that Jewish men were less itinerant overall in those times compared to the reality of Ashkenazi life in early medieval Europe, so polygamy may not have posed the same threats to women's economic security in the past.
Grossman highlights all the ways that Ashkenazi women asserted a more active role in religious life than they had previously played. Rabbeinu Gershom, who outlawed polygamy circa 1000 AD, was the first in a long line of Talmudic scholars in Mainz whose wives were heavily involved in the running of their schools. (The rabbi's wife made books, sewed Torah scrolls, hired tutors, and made clothing and food for the students.) Gershom's ban on polygamy was accompanied by a ruling that men could not divorce women without obtaining the woman's consent, indicating further how his synod was particularly concerned with protecting Ashkenazi women from abuse and neglect by their husbands.
I wrote an answer about the shift in Judaism from polygamy to monogamy (mostly!) here.