Somewhere around 20BC in the Roman Empire, the emperor Augustus Caesar wanted to to promote a bit of egalitarianism in Rome, and hoped to curb some of the conspicuous consumption of the rich. It turned out that the most conspicuous display of wealth was the freeing of slaves, usually in one's will. Slaves were quite valuable, and freeing a large lot of them on one's death was considered a great way to flaunt how rich one had been in life.
So, in the name of egalitarianism, Augustus set strict limits on the number of slaves that could be freed at any one time. Thus slavery was maintained in the name of egalitarianism.