After three major slave rebellions in sixty years (the last being led by Spartacus), the Romans noted that slaves could be kept under control by freeing a lot of them for faithful and profitable service. While this lost you your most competant slaves, it also eliminated the potential leaders of slave rebellions. The freed slaves were quite grateful, and were now loyal Roman citizens in the bargain. By the Medieval period, outright slavery had grown less popular, because serfdom and similar arrangements were found to be more effective.