Ocampa was a Kazon world. The Ocampa were vermin it was the Kazon's job to put down. Sometimes they trained this vermin to be slaves, but it really was a waste of time. Imagine what would happen if one random Starfleet Captain told the Klingons or Romulans that they were not allowed to keep slaves, and that all their slaves were free on only that Captain's say so and moral fortitude? Fleets would be sent to Earth to level the frakker until it looked like the 12th century.