Habitual Idiot, as it happens, there is 'another reason', an actual reason: really, Auster, in some ways, on some topics, you're at least as bad as the leftists.
Blacks in present-day America are "falling behind" academically not because of their biology, but because of cultural permissions and expectations. Blacks in present-day America disproportionately commit violent crime not because of their biology, but because of cultural permissions and expectations.
Blacks are human beings -- just like you whites (*) -- which means that, just like you all, they are 'fallen' (to use Christian terminology): human beings are lazy and perverse. When given permission to short-sightedly self-destruct, that's exactly what most humans will do.
For two or three full generations, now, the "liberals" and other leftists have been teaching black Americans that it is pointless for them to exert themselves, either at learning or at earning their own livings, and that they have every right to expect and demand "free stuff" because whites "owe them", and also, if the mood strikes them, that they have every right to bop that little old lady over there over the head and steal all that she has, or even to murder her. Given human nature, the results we constantly see, and that you're always on about, are exactly what one should expect.
But all this "human biodiversity" bullshit that you and your ilk are always on about doesn't address the issue: the problem is not biology/race, it is culture and sin. And you, claiming the name of Christ, surely ought to understand this.
(*) For rhetorical effect, I'm momentarily ignoring that I'm mostly white myself. And, I gotta tell ya', in my experience, most of you whites aren't any more on the ball, mentally, than you (Auster and ilk) are always going on about blacks.