from my very basic reading on the subject I believe the cause is a lack of black students coming through top universities. I have seen it stated that there are many black developers but many of them do not have degrees and learn to program outside of university. I found this particularly interesting because this is my background as well, I come from a nation less hung up on degrees and willing to accept experience as a substitute.

Corporations could help by considering interviewing developers who go to other universities or those with good experience but without degrees.