Theres a simple explanation for this. African Americans have victim and entitlement mentality due to the welfare state.Black Americans have a massive cultural and morality problem. I suspect they are not representative of the Negroid race. Most of the non-refugee Negroid migrants in the US, Canada, and UK are doing well. There's a selection bias going on, but still there's something particularly off with slave descent Negroids of the Western hemisphere which can't be extrapolated upon the entire Negroid race.
Also other black africans in USA tend to do better because well, they've not been trapped by the welfare state but also because of selective migration, we all know the Igbo tribe of Nigeria are probably the smartest black africans and those are the people permitted into America.