Within Ashkenazic circles, polygamy was prohibited by a ruling of Rabbi Gershom (960-1040 CE) that also prohibited various other things like reading other people's mail and divorcing women against their will. Note: This prohibition did not prohibit polygamy in cases of levirate marriage (if you were already married and your brother died without kids, you would have to marry your brother's widow in addition to your preexisting wife), and if you REALLY want to remarry but your wife is refusing to let you divorce her there is a clause that allows you to potentially remarry. Sephardic/Mizrachi circles permitted polygamy until relatively recently (Technically still does, but common practice is to have kethubahs explicitly prohibit the husband from taking a second wife), and essentially only prohibited it when people moved to the state of Israel.

Unfortunately there is no actual extant text for the ruling when Rabbi Gershom supposedly wrote this, so we only have reference to him and this ruling in quotes from other authors. The Shulchan Aruch (1563, Rabbi Joseph Karo), while technically a Sephardic text, lists a limit on how many wives one should marry with a reason (as long as you can support them, and are able to spend time with each one Within a month), which if you want, you could take as a hypothetical reason for why polygamy would have been abandoned.