Cultural factors, and honestly I wouldn't be surprised if there's some amount of racism there to, although in a more discrete way.
American culture tells black men that they can't make it in the corporate world (and the corporate world itself actively discriminates against black men), and thus they feel that things like either the NFL, NBA or rapping is their only way of "making" it. There is simply a smaller pool of whites playing football even at the highschool level, thus there are also going to be less of them making it to the top. Whites are probably less interested in things like football, basketball, etc because they aren't systematically oppressed in other parts of society (whereas blacks are), thus they don't feel the need to work their butt of to be the best football or basketball player while growing up as a kid.
On the other hand even *then* we make sure the best positions are given to whites, just as we do in so many other areas of society. The team may mostly be black but the Owner, the Coach and even the Quarterback is almost always white.
