Culture.

Cultures, like animals, compete for environment. Monogamy (which works every time!) is pervasive in culture, and so The Other must be banned and Evil. Thus, it is. You can come up with a bunch of reasons why, but in the end it's subjective, like everything else.

As to why it ended up being more successful culturally, my guess is because it's more socially permissible from a stability perspective. If every man has one wife, there's less conflict. If one man has five wifes and four men have no wives, there's more likely to be societal conflict. The same would be true in a female dominant society, though as evolutionary biologists have frequently pointed out, females bear children and men bear conflict. So from a cultural perspective, it makes sense to "tame" socially aggressive men by giving them more opportunities to bear offspring. For women, it may make sense to actually be with a man if he's relatively extremely wealthy/dominant to other men, even if he has multiple wives.

Mmmm... girls.