I doubt it's genetic, because there are black (from African descend) people with all kinds of muscular structures. They aren't all either skinny marathon runners or 100m sprinters or basketball players.
It can't be economics. Of course, many people in Africa may not be able to afford a race bike, and the roads may not really be ideal for it either. But what about the millions that live in Europe and the US? I hope we can pass the racist stereotype of them being poor, lazy and/or living in the ghetto's on crack. Of course there are more white people in Europe, but that can hardly be the explanation. Cycling in Belgium is about as big as football (soccer) is. Our national "soccer" team consists of 50% people of African descend. They weren't all born in the gutter and they weren't all **** poor and all of them were born here. You can get a 2nd hand race bike for 100 bucks if you don't mind riding with a 20 year old thing. I bought my Ridley (3 years ago, new) with campa Veloce/Centaur for 900 euro's as a winter promo. In Europe, most countries have good social security, making the argument even more insane, especially since cycling is so popular here.
So what? I believe it's purely cultural. It's simply not considered to be cool. 5 hours of racing and in the end only one of 200 can win. A lot less chance to diferentiate yourself from the others. Riding together for 2 hours, is a lot less social than it is playing soccer or basketball together for 2 hours. These things matter. You could argue that marathon runners suffer the same conditions, but i think there you should make a distinction between black people living in Africa, and those living in western countries. As has been said, the people in Africa just don't have any other option besides running and soccer. Even finding basketball fields would prove to be a problem, let alone a decent ball with a leveled floor. As opposed to running or soccer, which you can do anywhere, on any surface with any object (even a can). Maybe the black youths that live in the west (and especially Europe, since it's less popular in the US in general and where there have been more black people over a larger period of time), are still influenced by the sport their older family members practiced (who maybe still lived in Africa... hence, not cycling).
I would expect things to change over time.