If you want the actual historical answer, though, it's largely because the federal government stopped supporting Reconstruction policies, which meant there was no longer the necessary protection in place to allow it to function. And, as I said, white supremacists reclaimed control over the south.
My original point was that there's at least a case to be made that African Americans were better off during Reconstruction than they were 100 years later, which shows just how bad things were in the 20th century. It had nothing to do with the ride or the movie.