A number of reasons. Looking from the perspective of someone not from America.
1 - US has much more of a culture of sport for youngsters, both in and out of school.
2 - Wrestling in school. One of the cores of MMA available widely, with high quality coaching from an early age.
3 - Numbers. If you have 100 people competing for 1 thing, the best of the crop will be better than if you have 10 people competing for it.
4 - Money. There is far more money put into sport, including at school level, than in other countries.
5 - Location. UFC is an American organisation that trades primarily inside the US. American athletes have to sacrifice far less to compete.