Dear Friend,
I am neither a historian nor a biblical scholar, but I believe several factors were at work during those times. Probably the first was (sigh) male chauvanism. Since shortly after God created created Adam and Eve, men have been pointing to the fact that Adam was created first to assert a natural superiority over women. This would, of course, allow them to do what they wanted (e.g., have multiple wives) without any constraints from the "weaker sex".
But a more important factor, in my opinion, was procreation. The population of the world needed expanding so that man could, "...be fruitful and increase in number; multiply on the earth and increase upon it." (Genesis 9:7) The fact is that, while women can only bear one child at a time, men can deposit many "doses" of sperm (I really couldn't think of the right word, here) during any given time frame. Therefore, to maximize "productivity", the logical answer was to allow men to have more than one wife so that their contribution to population growth was maximized.
Also, it is an often overlooked fact that during ancient times, women were judged by the number of sons they bore; again, I believe, a population issue. Men were needed to till the soil, tend the flocks, and (unfortunately) fight the wars which cropped up occasionally. This tradition was taken to the extent that, if a woman's husband was killed before she bore children, it was considered that man's brother's responsibility to give the woman a son, and if that didn't work, his brother's brother, and (well, you get the picture). If a woman reached old age without having borne a son, she was treated almost as an outcast.
There is no doubt, I think, that God does not condone polygamy. Our Lord Himself said, "'Haven’t you read,' he replied, 'that at the beginning the Creator ‘made them male and female,’and said, ‘For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh’? So they are no longer two, but one flesh. Therefore what God has joined together, let no one separate.” (Matthew 19:4-6) And the Apostle, Paul, warned his protege', Timothy, that, "Now the overseer is to be above reproach, faithful to his wife, temperate, self-controlled..." (1 Timothy 3:2, emphasis added)
But God created us with free will. And free will has led us astray many, many times throughout history. This case, I believe, was one of them.
God bless you in your quest for knowledge.