It's more over how these athletes were raised with sports in America. I'm not trying to be racist at all, but the majority of blacks in America have grown up in neighborhoods or with friends that play basketball or football during a lot of their time, as have whites. There isn't a ton that the US as a whole can do about it really and I don't see a real reason to. The MLS/US National team still dominates over a lot of leagues/countries even though every time you walk into the streets you don't see kids playing soccer outside no matter where it is like many other countries.