Because they have Aryan blood. Niggers are extremely easy to defeat and colonize (see picrel), so they were conquered many times and through luck of the draw, you get some nigresses who have mostly Aryan genes expressing themselves, and many "blacks" who are 80 or 90% Aryan, the nigger blood giving the dark skin but the rest being Aryan.
The truth is, any "black" who succeeds in anything without being 100% propped up by jews is not a pure nigger, as pure niggers are all clinically retarded.