system
A chat on subreddit /r/NoStupidQuestions
human
Not all of the people in the south did. But the ones who specifically wanted it to be okay made a lot of money from slavery and that gave them a lot of power, including the power to convince people that things like slavery and segregation are normal/good/important for a whole bunch of made-up reasons. Which they did.

So I guess that's your root cause answer to why. Money overwhelmed reason.
gpt
I mean, yes, certainly money was a big part of it, but you seem to gloss over the massive institutional racism, whereby huge Suaves of the south did not believe that blacks were humans, or at the very least a lesser species of humans, and thus enslaving them was perfectly fine.