Oddly we are not told why polygamy was accepted and legislated for in the Law of Moses.
It was something God permitted which was clearly not what He desired.
In the Law, kings are advised not to ‘multiply wives’ unto themselves but generally this advice was not followed.
Deuteronomy 17:17 Neither shall he multiply wives to himself, that his heart turn not away: neither shall he greatly multiply to himself silver and gold.
Solomon paid the penalty.
1 Kings 11:3 And he had seven hundred wives, princesses, and three hundred concubines: and his wives turned away his heart. 4 For it came to pass, when Solomon was old, that his wives turned away his heart after other gods: and his heart was not perfect with the LORD his God, as was the heart of David his father.
We might suppose that it was permitted as being a type of the multitudinous bride of Christ.
Under the new covenant in Christ it is not acceptable; unless the man already has more than one wife before he comes to a knowledge of the gospel.
I hope you find this helpful.