I think a lot of stuff in the Old Testament is like that. God knew that those people could not live up to his ideal, so he gave laws regulating their behavior as best he could. Another example is when Israel asked for a king. God knew it wasn't the ideal, but Israel was bound and determined to have a king (after all, all the other nations were doing it!), so he allowed them to have one.
God was dealing with real people in a real cultural milieu of a particular time and place. People are not just a blank slate, or a lump of clay that can be shaped into just anything. People have their own wills, their own personalities and limitations. God was working with the material he had to work with--i.e. these people with their own cultural prejudices and traditions. So God took these people with the cultural traditions of their times and did the best he could to shape them into something decent. Slavery was a part of the cultures of those times, and I'm sure God knew that those people were not about to give that up ("Come on Lord--all the other nations are doing it!"). So God took those institutions and gave laws to make them as fair as possible.
The Bible (especially the Old Testament) is full of messy stuff such as slavery and polygamy. Where did this stuff come from? From humans. They are works of human sinfulness. Because of human free will, God could not just snap his fingers and get rid of all the imperfections in a culture made up of imperfect humans. He had to work with the material available.
Over time, of course, God's people have learned to give up many of these questionable practices. For example, in the New Testament, wars and polygamy are all but absent. Women were given much more equality. Slavery was still there, but eventually Christians outgrew that too--and helped their cultures outgrow them. After all, Christians were in the forefront of the abolitionist movements in Britain and the United States.