Because of the peculiarities of the slavery and the slave trade back in the 17 and 1800s. In the South, plantations were huge, they had large communities in them which were completely separated from the manor houses of their masters.
People always forget is how utterly huge the US is. Well, the South is as big as the entirety of continental France, Germany, and the United Kingdom put together. The United States was still organized along Western European governing principles and simply couldn’t apply the kinds of governance that their compatriots on the continent could, so a lot of it was just left to its own devices.
This is the primary reason why Europe looks more advanced in terms of welfare and social issues than the US. There’s an organizational cost to strong regulation and government that only grows as the number of people and land area does.
So there were a lot of slaves. They all had their own communities before they were emancipated, and for protection and support, they kept to those communities after they were emancipated. Sure, there was some movement, but people generally tend to stick to what they know.
This is where that other aspect of slavery where the US differed greatly from other nations on. Slaves were black, and free citizens were white. People are impressionable and if they live their entire lives under a regime, they’ll find ways to justify and protect it. If you grew up your entire life seeing and being taught that if you see a black person, that person is just a slave. Freeing them all did not free them from the social status of former slaves.
Other areas of the world weren’t necessarily like this. Slaves throughout antiquity often looked exactly like their masters. Alternative ways of identifying them had to be devised, most often clothing. When the slaves were set free, all you had to do was change your clothes and you suddenly were treated like a citizen.
But in the US, slaves were black, and citizens were white. You can’t just change your clothes to escape persecution. Free blacks in the North would get captured by scoundrels and sold to plantation owners in the South, and nobody thought much of it. Your skin color marked you and you couldn’t do anything about it.
After the Civil War and emancipation, it’s not like they just pulled out a seat at the table for former slaves. Black Americans were shut out of the political process, given the worst jobs, expected to accept their lot in life or even demanded to go back to Africa. (A few did and they started the African nation of Liberia. I think the similarity of the name to “Siberia” isn’t entirely coincidental)
This stuff takes generations to undo. Our society is finally at a point where a plurality of people recognize racism, but that by itself doesn’t eliminate racism. All of those little things that white people grew up with, the completely ‘normal’ subjugation of black people has to be examined and rejected.
All because we wanted cheap cotton T-shirts.