You may as well ask why africans aren't any good at snooker, or darts, or go-karting.
The answer is a range of practical and cultural issues. In cycling this is down to availability and cost of equipment, difficult training environments and the lack of a well-developed competitive structure (which, itself, is due to low demand for it because of the preceding reasons).
No reason why africans shouldn't be good at many many other sports too but, up to now, the opportunities to grow participation and competitive / support organisations to a level where they can compete with nations / continents with a 50+ years head-start isn't there.