“Why did God allow Jacob, David and others to have more than one wife? Did God merely overlook their ignorance (Acts 17:30)? Surely they knew God’s law on marriage. As Jesus said (Matthew 19), from the beginning it was one man and one woman for life.”
The first instance of plural wives is found in Genesis 4:19, “Lamech married two women, one named Adah and the other Zillah.” We are not told whether or not that was with God’s approval. But from that time, it became more and more common for men to take more than one wife. The more important or wealthy the man, the more wives he could afford. So, Jacob had two wives and two concubines. David had several wives as the king over Israel. Solomon took this to extraordinary lengths with hundreds of wives and concubines (1 Kings 11:3).
However, you are right about God’s original intentions . . .
“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 (New International Version)
As Jesus discusses marriages, He takes His audience back to the original purpose of God. God joined them together and humanity should not alter that arrangement. What was God’s view of those multiple marriages? My best understanding is what Jesus goes on to say about divorce . . .
“Why then,” they asked, “did Moses command that a man give his wife a certificate of divorce and send her away?” Jesus replied, “Moses permitted you to divorce your wives because your hearts were hard. But it was not this way from the beginning.” – Matthew 19:7, 8
Jesus doesn’t say God approved of the Jews’ attitude toward divorce but that God put up with their hardness of heart. I believe the same principle would apply to the multiple marriages you mention. God did not approve but tolerated their actions, in spite of the fact they were not what He originally created.
Other factors are important to notice as well. I can find no instance of multiple wives in which there were no serious family problems. Jacob certainly struggled with his wives and sons due to the nature of his marriages. David had problems with his wives. Solomon was led to tolerate idolatry by his wives.
In every instance where a man of God took more than one wife, there were serious family problems that followed. The reason is really very simple. It was not God’s purpose nor intention that we should have more than one spouse. When we violate God’s law, we suffer because we were not created that way.