...if not lack of interest, what's your explanation for why America isn't top dog in soccer? It is the third largest country in the world, and the two above it are more racially homogeneous and much less wealthy. The US has over 300M people, lots of money, is very multiethnic, has a strong sports culture...does this sound like the profile of a country that would be bad at soccer? Doesn't to me. It sounds like social reasons--i.e. lack of interest--plays a the main role in why they don't do well.
That will probably change in the next decade or so, though, as the ever increasing number of Latin American immigrants popularize soccer.