This is completely untrue. The fact is, that SOME members of the black community underachieve, but, that is the same within any ethnic group.
The sad fact is, that the black community have been subjected to racism; most notably in Apartheid, due to the insularity of the world, which sadly is still reflected today. Plus, it's similar to saying that why are all Asian and Indians doctors? Some cultures excel in certain fields whereas others do not.
But, the fact remains that the black population are no less intelligent in comparison to any other cultural or ethnic background. If anything, the black community are the most humbling and generous people that people in today's society know.
So, in my opinion, what you see is a microcosm; open your eyes, observe the macrocosm of society and you will see that every background is just as gifted as the other.