Polygamy began to end when agricultural development and settling got introduced. During the nomadic non-agricultural period, polygamy was common and people were not tied to a single person.
Polygamy and agriculture were intertwined because when agriculture arose there were certain people who could afford to feed more wives and hare more social power in general - this lead to a state where they could afford to be in "command" of multiple people through this - food at the time was essential namely!
