I don't get the impression Buddha and Jesus really were challenging the political order. Most likely they saw slavery as part of the political order and not in and of itself a moral issue. If you're going to discuss the morality of slavery you might as well discuss the morality of inherited or absolute leadership.
It was through pure strength of public will that the West divulged into Constitutional Monarchies and we owe all that to the age of enlightenment and the renaissance period.
Sometimes I think most people don't realise how important those periods were for social change, more important imo if not in fact than any period before or after.