I agree that Lyndon Baines Johnson was the final nail in the coffin, but I think the start of it was Jim Crow laws and the Wilsonian enforced separation of the races.
Before the legal separations, black people were incorporating themselves into the mainstream society. That stopped with Jim Crow and segregation. But they still interfaced with the main stream in their jobs and shopping. So they still had an example of what was possible if you worked hard, and did right.
Johnson made it possible for some people to live their whole lives without seeing a civilized, function society (except for the court system). He cut the final tie with the “Great Society” programs.
It wasn’t just Johnson; it was the Black Panthers, other militant movements, various other organizations that sprung up.