"In bygone eras that it was once the duty of the ruler to lead their men in battle, in other words when their country went to war the rulers would be there in the thick of it and due to this there was always the probability that they could end up dead, saying this when there sons became of age it was also expected that they would too follow their fathers into battle. Now given this it was almost impossible to be assured (given the lack of any medical advances to ascertain sex of the baby before birth)that one women could keep giving birth to a male heir in case the king and his eldest sons were killed in battle, so it was common sense to have more than one wife as this increased the chances of more male heirs, this theory spilled over into the higher class citizen (the wealthier men of society) populations as well.
see:
Why is a man allowed to have more than one wife in Islam?
URL: http://www.qatarliving.com/node/408861
"In bygone eras that it was once the duty of the ruler to lead their men in battle, in other words when their country went to war the rulers would be there in the thick of it and due to this there was always the probability that they could end up dead, saying this when there sons became of age it was also expected that they would too follow their fathers into battle. Now given this it was almost impossible to be assured (given the lack of any medical advances to ascertain sex of the baby before birth)that one women could keep giving birth to a male heir in case the king and his eldest sons were killed in battle, so it was common sense to have more than one wife as this increased the chances of more male heirs, this theory spilled over into the higher class citizen (the wealthier men of society) populations as well.