Polygamy was the most common form of marriage in those times. The main reason for it was the lack of men. Women probably outnumbered men two to one in some cultures.
Depending on how warlike a tribe or society was, they could lose up to 40% of their males in their prime to violence, war and hunting accidents.
Anthropologists base that on Stone and Bronze Age artifacts, graves and also, on tribes of people in recent times who lived in Stone Age or Bronze Age isolation, like the natives of New Guinea.
They had similar problems, and they were polygamist...so were many Native American plains tribes polygamist for that same reason or the Mongols. Warrior cultures lose a lot of their men. Yes, the Hebrews were a tribal warrior culture in those times. Have you read their book...hellooo? They're always fighting with some other tribe who wants their goats and gardens. Foreskin trophies and heads on pikes and swording women and children and decapitating and...well, they definitely get an R rating from me.
Also, high infant mortality, diesease, famine...life in the Bronze Age was kind of short, violent and not a whole lot of fun sometimes. Probably why their gods were mean SOBs most of the time, Jehovah included.