I asked a pastor about this one time. He said something like just with individuals, God wants humanity to get rid of some sins first, then He can help them with the others. Polygamy wasn’t a priority back then. The big one way back when was worshiping other gods. Of course marrying a bunch of women with different beliefs didn’t help - so they sort of went hand-in-hand if you ask me!
God is also far more concerned with divorce and fatherlessness.
this does not yield a consistent model of g*d’s intention as expressed in differences between the old and new testaments. for example, pork is interpreted as OK to eat in the new testament but it is not kosher in the old.
“Of course marrying a bunch of women with different beliefs didn’t help”