Soccer and hockey require some very specific skills. Its kind of like hitting a baseball. Being a speed/explosive athlete doesn't hurt, but it doesn't really guarantee success.
Some people bemoan/think that blacks are under-represented in baseball because they dominate football and basketball, but baseball is a very different game. If "athleticism" is kind of plateaued in baseball, then there is no reason to expect blacks being represented at a rate/percentage greater than the general population. In terms of specific strategy and baseball, the stolen base is pretty worthless and MLB managers have FINALLY figured out that it makes sense to position your fielders in response to the specific batter...so a general athletic trait like "speed" isn't all that desireable in the MLB.
Soccer in the US is funny to me. So much of the game is skill on the ball and overall conditioning. The US public just thinks we can put 11 black guys on the field and dominate the game, but speed/explosion is just a small part of excelling at soccer. You have countries that put their entire male population in soccer academies at age 4. The skill level they acquire is unbelievable.
Compare that to football, where simply being big/strong/fast can make you a great player. So many success stories of guys who didn't start playing football until their junior or senior year in high school and then went on to a HOF career in the NFL. If you are a soccer player and you don't start playing soccer until high school, you are about 10-15 years behind the European and South American players.