People evolved differently due to climate, available resources, connection with nearby tribes, natural defensible borders, etc. People who lived in regions with seasons had to learn how to plan ahead for winter, build shelters, form family bonds during extended periods of closeness. Borders and food abundance in Europe allowed many peoples to maintain sovereignty while still having close interactions with others.
Culture is vastly more important than most people realize, and just as much a product of evolution. The formation of large functional governments with wide support of the people, elections, complicated legal systems, truth based religions, etc vs tribal justice, might makes right, and and superstitious nonsense.
People are adaptable and can integrate, however instead of slowly integrating them like the south wanted, they were suddenly thrust out without jobs, lives, homes, knowledge. And once they did start progressing, corporations, drugs, and welfare destroyed as much as they could. Corporations mostly for greed and not outright racism, as they also obliterated the locally owned business for whites as well. Drugs, gangs, and wildly unpopular police crackdowns, despite being the only thing that helped. And yes, welfare was a huge problem as government agents went around door to door to tell black women they could qualify as long as they left their man. Single mothers need help! How could you not support them?!
And now there is a culture where blacks are literally given everything. Apartments, food, electricity, internet, clothes- all without ever having to care about it. No stipulations. Piss in the corner, not your property. Do laundry? "That's white people shit" throw that shirt in the TRASH, just go get another on the government dime. Without a culture of their own, some made one out of hating the white slaver without realizing that's not even a culture. You specifically have to avoid doing anything "white" like education.
