A common confusion between the Peculiar Institution and classical slavery.

Nobody in the ancient world was enslaved *because* of their race. (I have had to actually explain to people that the slaves in Rome were of all colors).

The thing about southern chattel slavery was that it was not slavery in the classical sense. It was a caste system in which the lowest caste were property, combining features of the two.

And caste systems are very hard to get rid of.

We might not be allowed to buy and sell "lower caste" people any more, but the caste system is still there.