God made the first marriage. Adam married one woman - eve.
At no time were we directed to change that up.
Marriage to multiple wives, and use of concubines, is noted in the Old Testament. That doesn’t mean it is endorsed.
Noah had one wife. Abraham had one but used a concubine/maid at her behest. Isaac had one wife. Jacob had two, and used two maids. David had several. Moses had one.
Jesus in referring to marriage speaks of one husband and one wife. Laws refer to one. The epistles when giving directions about marriage assume one.
In short the prescription and the norm as well as the standard for eldership is one.
Polygamists in the Bible
http://www.biblicalpolygamy.com/polygamists/
That didn't end well.
Christianity’s often been flexible on the subject. For example German’s tribes who accepted Christianity were allowed to keep their multiple wives but no future extra person marriages allowed. It’s one of those things that happens from time to time that’s not ideal so it’s suppressed without breaking up existing structures.