Wage labor - and a class of workers who have nothing to sell but their labor - is a part of what defines capitalism. I mean, I don’t want to be so rigid as to say the economic system in the South was nothing like capitalism though, these things don’t always have firm boundaries.

Edit: I’m literally a communist, in no way should my explanation be considered “defending” capitalism. Chattel slavery and capitalism were inexorably entwined together, I’m just getting into the weeds on definitions.