Distance running is the national sport in Kenya. They spend money on it like the US spends money on football. Of course, not to quite the same degree, but the point is still that it is their national sports focus. If Kenyan's are genetically superior at marathons, then Americans are genetically superior at football.

Countries as large as the US, India, China, etc., have large enough populations that someone in them should match the "genetic ideal" to excel at a certain sport. It is economics that prevents them from doing so. Either their country or culture doesn't value the sport and thus doesn't spend money on it, or they must spend too much time earning to feed themselves that they can't train.

Or that particular person has just never tried the sport. Or there are people who are actually more genetically suited to the particular sport but are discouraged from trying because they hear the current theories on what makes a genetically suitable athlete for that sport.

All we know is that Kenya as a country has the ability to produce excellent marathon runners. To heap all of that on "genetics" is grossly short-sited.