Labor shortage and contradictions involving extended supply chains. Eventually you run out of slaves and your smaller regional landowners figure out they can save money by making their underclasses reproduce with one another. Just bind them legally to your land and there you go. That's cheaper than constantly funding military expeditions, training the slaves, and then working them to death just to go catch more slaves.

Fun fact, the world "proletariat" comes from the Latin word "prole" which means offspring. Like it's your underclass that reproduces itself.