Because their ruling classes were not as much controlled by the Slavers and because of Lincoln's political adroitness in heading them off. They had always been less controlled by the RAT party throughout their history which warded off some of the baleful influences later to destroy the others.
The real issue, is there were just a different set of morals back then. Generally people were just rotten to each other. Most of the country stayed drunk at any given time, they enjoyed public executions, blacks too, owned slaves and as in general treated their slaves worse than the white owners. The problem basically stimmed from Africa itself by the wholesale selling of slaves. The ones they couldn't sell, they killed. We contributed to it by having a market available here and arming the more powerful tribes. These problems still go on in Africa today. That is basically where the problems started and still exist.