Because the south depended so much on slavery for economic stability, it was at an extreme disadvantage with the north economically. If the southern economy had been more diverse, slavery would have been easier to give up. Even if farming in the south had been more mechanized than it was, slavery would have been easier to dispense with.
Economic differences created divergent interests. The slaveholding South, with an economy based on cash crop exports, had different interests than the more diversified North, which was in the process of industrializing. Most historians now argue that the South's reasons for attempting to expand slavery into the territories, the immediate cause of the political crises of mid-century, went well beyond simple economic differences, however. But certainly they were the fundamental reason for sectional discord.