Polygamy originally only stopped in the New Testament because of Greek/Roman law
In the Old Testament it was common for the main characters to have multiple wives. It is my opinion that this completely stopped in the New Testament because polygamy was illegal under Greek and Roman marriage law. This change happened before Jesus and Paul started preaching. So it was initially due to legal reasons rather than God first changing his mind.
Characters in the Old Testament that apparently had multiple wives included Lamech, Abraham, Nahor, Esau, Jacob, Moses, Gideon, Elkanah, Saul, David, Solomon, Ahab, Rehoboam, Abijah, Joash, and Ahasuerus. King Solomon was said to have had 700 wives and 300 concubines. The Bible says Solomon's foreign wives led him astray but he could have just restricted himself to Jewish wives and remember that a single wife can also lead you astray. Despite this I think Solomon was still pretty close to God since he built the Temple and wrote three books of the Bible.
Deuteronomy 17:17 says "The king must not have many wives. If he does, they will lead him astray. He must not store up large amounts of silver and gold". There doesn't seem to be a punishment for these things. Solomon also broke the rule about kings storing a lot of gold - 1 Kings 10:14 says "Each year Solomon received 25 tons of gold" (666 talents).
If God wanted men to always be monogamous I wonder why humans are also subject to the Coolidge Effect. In animals this involves males ejaculating a lot faster if they are presented with different females compared to having to repeatedly mate the same female. This explains why men can prefer novel porn rather than only having images and videos of a single female. In polygamy the man can have multiple partners and so express the Coolidge Effect in a way God permitted.
Maybe God prefers monogamy but he still permitted polygamy in the Old Testament. And if it wasn't for Greek/Roman law I think God would also permit it in the New Testament - like he did for slavery.