I believe they are dominate almost in every olympics and in all of the sports because of there school system, the facilities of the middle schools, high schools and colleges are amazing. This gives the opportunity to the kid to go out there and do any sport.
Like anywhere in the world, if the facilities are present and manage anyone could do well with their sports of choice. The schools in the US are competitive as long as you have the MONEY so most likely you will succeed to to whatever sports you select and master in due time. There are also a lot of hidden poverties in the United States in every corner states white and black populations so it has nothing to do with just poverty at present but culture, individualism, and specialty.
Example: Michael Jordan is a dollar billionaire legend in Basketball but failure in Golf because US residents aren’t design to do both.