There was a rabbinic ban on polygamy in the 1000s C.E. Most Jews followed the ban although some sephardic Jews did not. But even among Jews that did not acknowledge the ban, polygamy was never very popular.
faqs.org/faqs/judaism/FAQ/04-Observance/section-55.html :
About the year 1000, Rabbi Gershom ben Judah issued an edict (Herem
de-Rabbenu Gershom) which was considered authoritative among Ashkenazi
Jews. This edict substantially prohibited plural marriage. One
exception was allowed: A man could marry more than one wife if he
obtained the special permission of 100 rabbis in 3 countries.
Originally, Gershom’s ban was limited in time to the year 1260,
however it has continued to be accepted by Jews of Europe and the
Western World to this day. Rabbi Gershom’s edict was followed less in
sephardic countries: cases of polygamy were found in Spain as late as
the 14th cent. That such cases were not rare may be inferred from the
fact that in the Spanish communities the Ketubah, the document marking
the betrothal, exacted that the man was not to take a second wife. The
Islamic influence on the Jews in Spain was more or less pronounced
until the expulsion at the end of the 15th cent.