I'm going to go out on a limb and say that I think it's a matter of culture. Traditional classical has always been, not a white "person's" music, but the white "culture" music, (or, PC: European). Whereas, solely because of their culture of origin, black "folk" have primarily been exposed to gospel, jazz, rap, and other forms of "black" music. I think it's mostly about exposure, and this is a function of culture of origin.
I, on the other hand, grew up listening to very little music. My parents did not have a stereo, and we didn't listen to the radio in the car. I didn't strart regularly listening to music until I was in high school, and then I was drawn to "oldies" (60s-70s). (I'm white, by the way).
Now that I'm old (39 next month), my tastes are very eclectic, and largely defined by how long I can listen to a certain style before I've had my fill. I have a very short attention span for rap, but recently was listening almost exclusively to jazz and gospel ("black" gospel, not "southern" for which my tolerance is roughly akin to rap). Then the gospel radio station in town reformatted and I found myself less interested in jazz, and now listen to a mixture of classic and progressive rock and some symphonic classical as well (when I can concentrate on it).
So, in short (too late), the reason very few blacks compose classical music is exactly the same reason so many blacks do "compose" rap, R&B, hip-hop, jazz, etc. It has to do with the very strong pull of one's culture of origin.
My $0.02.
