It's much easier to develop your basketball skills in American than your football skills. They've got rims every where and street ball exists in every city. It's just like the Brazilians and how the kids play football on the streets. Any good, athletic American kid is drafted into his JV/Varsity basketball/american football teams and that's it, there's no chance to develop them into a good footballer. I also think the fact that they've got to be drafted and shit, and thus not allowing professional teams to develop their own talent from a young age (like 12 or 13, like it is in Europe) means the quality of the players is much lower than you'd expect from a country with such a large population and good sporting infrastructure.