JW would answer it was in order to fulfill God’s purpose. Polygamy was tolerated in order for the Israelites to grow fast to a big nation. I think that’s the explanation I read once in the WT.
Yes that is the explanation I heard.
But think about it, polygamy of itself, does not cause the population to increase any faster.
About 50% of people are female, now those female can became pregnant by the other 50% of the population or they all can become impregnated by one male. Either way, they can't pump babies out any faster.
But if you steal or buy women from other tribes, then yes, you can make your tribe grow much faster due to the much higher female to male ratio.
So the JW excuse is dependant on stealing/buying foreign wimmen.