The "slavery wasn't so bad, maybe we should bring it back" idea basically comes from the belief the black people can't or won't take care of themselves and that all the secret welfare they get just encourages them to be even more lazy than they'd otherwise be. So perhaps they would just be better off if they were taken care of by the benevolent Great White Father who would "encourage" them to fight their basic nature and work a bit, while making sure they were well taken care of.
Racist and more than a little ignorant of the reality of slavery, but there you go.
