'Polygamy' on its own was not really encouraged. I think I can explain. Jewish culture stressed not placing value upon bloodline, upon not confusing blood with importance, spirituality with beauty. The Jewish scriptures are full of stories of great people who have rotten children, on purpose.
Marriages were arranged (and still are among some Jews). People today wonder why marriages were ever arranged, but it seems it was an article of faith much like circumcision was/is. Man was 'Given' his wife to symbolize that faithful children would come from God and not from the man's lineage -- his particular sperm or his particular spirituality. (This was in heavy contrast to the attitudes of the notions in surrounding nations who thought people ought to be bread like horses for strength and intelligence. Yet another reason for Nazis and ancient Egyptians to hate Jewish culture by-the-way.) A wife was chosen for a man. Through her God would give righteous children which were the point of the marriage. If there were no children only then a second wife was required. Jews were very serious about their tokens of which circumcision is a good example. Arranged marriage seems like another. Note that Jacob, the only Patriarch to choose his own bride, wandered into error by marrying the wrong woman at first. Abraham, Isaac, Jacob and Joseph had awesome children from the ones that came from wives they didn't choose for themselves. They were given one good wife each specifically to provide a miraculous channel for God to bestow righteous children with as little interference from the man's own will as possible, so I think.
Supporting scriptures:
Genesis 24:4 "...get a wife for my son Isaac" (Note it does not say 'Wives for my son' but 'Wife')
Genesis 30:9 "When Leah saw that she had stopped having children, she took her servant Zilpah and gave her to Jacob as a wife."
Deuteronomy 25:5 --- A man was required to marry his brothers widow if his brother had no children, and the first child would belong to his brother and be his brother's heir. If he refused he was discredited with the title "House of the Unsandaled." This had very negative implications for a person in a culture which yearly celebrated a Passover recounting a 40 year journey in which their sandals never wore out. (Perhaps it meant his own children would not be considered Jewish. Don't know, but it was a bad title to be given.)
Genesis 6:2-5 -- It is difficult reading, because it talks about the 'Sons of God' marrying whomever they wished. Its a heavily debated verse, but the idea here is that only God can make spiritual children. Then in verse six it points out Noah, a man with only one wife, as a righteous man and contrasts him with all other men. He was found to be 'Righteous', and the implication is that he was not marrying whomever he wished.
Proverbs 18:22 "He who finds a wife finds what is good and receives favor from the LORD." This is likely a reference to matchmaking -- (I'm guessing). Here it should be noted that 'Wife' and 'Thing' are in the singular, and that literally the phrase could be spoken "Matsa Ishshah Matsa" or as they sing in Fiddler on the Roof "Matchmaker Matchmaker make me a match, find me a find, catch me a catch." A wife then, was a find that you did not find for yourself. (The sad characters in the play 'Fiddler on the Roof' don't seem to know why they're supposed to accept made matches, only that tradition directs that they should but its a comedy partly for that reason.) Through it all there is an awareness that the person given may in fact not be a 'Find', that the marriage may not work out; but it is hoped that it will. That is why the matchmaker has high honour when their matches are successful and result in awesome children.