The list of why the U.S. doesn't produce more world class marathoners is a long one. But here are 6 of the reasons I'd start with:
1) Emphasis in Youth participation in the sport of running is largely focused on sprints and middle distance running. Most all races for kids are short in nature and do not go beyond 5k until they reach college. So the vast majority of youth athletes train for these shorter races, with a greater emphasis on VO2 Max and speed development rather than the aerobic development which key to the longer races.
2) The U.S. High School and Collegiate system is very race heavy for a distance runner. There is a race every 1-2 weeks (in high school even twice a week) all through the school year. The emphasis is on racing and not training. Again this leads to a shorter distance focus where that can be more successful. It takes long periods of dedicated training to develop the best in longer distances.
3) The U.S. High School and Collegiate System are short term focused. The coaches get to work with the runner for between 1 and 4 years and that coupled with the shorter event focus described above (in 1 and 2), often leads to the coaches focusing where they can get the quickest benefit rather than what is best for the runner’s long term development. I don’t blame the coaches for this but rather the system that forces them to act in this way.
4) The coach’s education system is need of a major overhaul in the U.S. We still have instructors who teach young coaches that “young distance runners are like tires, they only have so much tread on them before they are done, so use the mileage sparingly.” – that is an actual direct quote from an instructor at a USATF coaching conference in 2010.
5) There is very little atlternatives for the kids who want to run longer distances and not follow the school system for running. Not many clubs or even fewer coaches to coach longer distance kid runners. Kids and their coaches and parents who buck this school system of middle distance focus and over racing, are mocked and chastised by coaches and parents in the system for not taking part in the system, making them less likely to do it even if it is in the best longer term development interest of the kid.
6) Many of the top runners in the country will never have the opportunity to train for or race a half marathon or longer distance until their mid-20’s. They may be at their peak physically before they even realize they have a talent for or take a liking to a distance that takes many years to work your way into. This leaves very little time to progression in the event.