Technological advances and efficiency. Must be a city-boy - visit a farm and you will understand.
I've spent plenty of time on farms. I know all about mechanization. I also know that the mechanical cotton harvester wasn't in use until about 1950, which is, not so coincidentally, when sharecropping--the agricultural labor system that replaced slavery in the south--pretty much died. So are you saying that blacks would have endured another hundred years of slavery before the south got around to freeing them on their own time?