Why can a man no longer have two wives?

I think Matt 19:1-9, though primarily about divorce and remarriage, is helpful. Under the Law of Moses, men were permitted to have more than one wife but this, like divorce, was because of “the hardness of your hearts” (v8); this was not how it was intended in the beginning. In the beginning God created male and female to compliment each other and be joined as “one flesh” in a life-long relationship (v4-6). This “one flesh” relationship can only be shared between two people, male and female – it is not possible to extend it another person.

So the intention from the beginning was for one husband to have one wive, and this is the model that Christians should follow.