Because, again, 400 years of institutionalized racism. And when they are successful, white people tried to destroy them, from burning down Tulsa in 1921 to framing Michael Jackson.
American Negroes are more affluent than Negroes in Africa and the Caribbean. That is because they have enjoyed the benefits of white civilization.
They tend to be less affluent than whites and Orientals in the United States because human evolution has not prepared most of them for the intellectual and social demands of civilization.
What happened in Tulsa happened a long time ago. Currently successful Negroes are celebrated by whites.