For any of a number of reasons, some of them ideological, but as many economic, historical, and religious. The South had been settled in colonial times by a different religious culture than the North. The latter's agrarian culture diverged increasingly from the North's industrialized urban ethic. And the feudal aristocracy that pervaded the antebellum South established a caste system that chafed on the egalitarian sensibilities of the North.
The Southern system was supported on slavery simply out of economic necessity. But it is a gross oversimplification to attribute all differences between the two regions to slavery alone.