I heard that polygamy was the result of a relative overabundance of women over men due to warfare which greatly thinned out the men. I don’t know if that is true, but it sounds reasonable.
In small, warrior tribal societies that may very well be true. I don't know how well it applies to more developed civilizations.
In general, in the ancient world kings could have more than one wife. It was a way of solidifying alliances. Nobles could imitate the royal practice, especially if their first wife couldn't provide an heir, but commoners tended to be monogamous.
I did find this on Wikipedia, though:
"On February 14, 1650, the parliament at Nürnberg decreed that, because so many men were killed during the Thirty Years’ War, the churches for the following ten years could not admit any man under the age of 60 into a monastery. Priests and ministers not bound by any monastery were allowed to marry. Lastly, the decree stated that every man was allowed to marry up to ten women. The men were admonished to behave honorably, provide for their wives properly, and prevent animosity among them."
Don't know if it's true or not or if any guys took them up on it.
Lately the BBC has been saying that politicians in Afghanistan and Iraq (where polygamy is legal) want to see more of it because of war losses.
A society in which that many men (enough to justify widespread polygyny) had died due to warfare would be on its last legs. Usually, they would be the losers of the war (hence no need to practice polygyny since the victors would be leading them all away to slavery).
Polygyny was never practiced by more than a tiny percentage of any society - by the "1%" of non-Western societies.
Regards,
It is also a result of the “bride price” system where men essentially “buy” their women from the bride’s family. Which, of course, brings us back to that part about polygamy being for the elite males. Non-elite males are limited one or no wife.
In the “dowry” system, where the bride’s family has to pay the groom, a some families end up sending their daughter to the nunnery.