(1) Selection bias. The pool of immigrants who make it to America in the first place likely skews well above average.

(2) Culture/ecosystem problems in black communities, even if it's politically incorrect to argue it. Black neighborhoods are plagued by gang culture/violence, drugs, patriarchal absenteeism (estimates vary, but roughly 50% seems likely), intergenerational educational deficit (if parents are illiterate/undereducated, their children are likely to be so as well), and various other more generic issues that stem from chronic poverty.