After the 10 Commandments, Why Were There Multiple Marriages?

Why were there multiple marriages in Israel after the giving of the Ten Commandments?

The fact of the matter was that while polygamy was contrary to God’s intention and ideal, nevertheless, because of what Christ called “the hardness of men’s hearts” (Matthew 19:8). This practice was tolerated especially in the case of a political leader whose dynasty would fail if he were unable to produce a son by his first wife.

–Encyclopedia of Bible Difficulties