Because the sport isn't that popular in the US. My country (Argentina) has produced a lot of top players in Soccer, and we export several players to the European leagues every year (and, many will argue, the top player in the world used to be Maradona until we replaced him with Messi). That is because people dream of being professional soccer players, they spend the day watching and playing soccer, just about everyone in the country plays soccer in their free time, so their kids grow up with that, the first gift a kid receives is a soccer ball, his after school activity is playing soccer, etc. If the kid shows any kind of talent, the parents will most likely try to get him into the professional circuit as soon as possible, and it's easy to get tried out and picked up by any club. In every city there are clubs and stadiums. Car racing is also huge here, and we have Fangio, who arguably is to F1 what Giacomo Agostini is to MotoGP. Meanwhile, we've only produced one international Basketball star, only a few in Tennis, and move to the Olympics, and we have no runners, no swimmers, etc. Meanwhile, when Boxing was HUGE here, we produced lots of champions, now that it's not so popular anymore, not so much.
Meanwhile, the US has produced almost all of the Basketball, Baseball or Football stars in history, while you import your Hockey players from Canada.
At the current level of most professional sports (and, the more popular they are, the better funded they get, the more competitive they are), it's not everyone that gets there. Liking that sport and being good at it is not enough like it used to be. Michael Phelps is not just a good swimmer, he is genetically predisposed for it, he is the 0.1% of the population. And that's not good enough either: It requires constant training since you're a kid.
In Italy and Spain, they breathe motorcycle racing. There are tracks everywhere, and people watch MotoGP religiously. This beasts got a motorcycle as their first gift, and they've been riding since they were 2 years old. And as soon as they could, they were competing. And their parents dreamed of seeing them make it in MotoGP. Many of their parents were racers too.
It's a simple matter of statistics. There is a small percentage of the population that have the alien gene, it's only a matter of finding that needle in the population haystack. The more people that practice a sport, the larger the chances of finding the gifted ones.