Coolblue has it right - soccer is something that parent's sign their kids up for at a young age here, but very few follow through, i think. a lot of kids opt. out to play baseball or football or basketball - the more traditionally american sports.
i loved playing soccer as a kid, but by the time i hit double digits i was playing baseball and football, not soccer.
soccer would have to become a more popular sport in america before that trend changed much.
it's really a shame as well, because kids stop playing about the age they'd start actually improving their game and becoming legitimately talented.
most kids have stopped by high school, and if you dont play in high school, you probably arent going to play in college and by then you've (basically) missed your chance.
at my college, most of the decent players on the men's soccer team are either of mexican descent, or african international students. there's maybe two or three decent white boys.