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
Not just money. Treating people of color as a separate class of people not worthy of the rights and freedoms and privileges of other Americans didn't end with slavery.