You cannot overlook the fact that distance running in the US has so much competition with other sports. The US is wealthy enough where high schools fund numerous sports, such as football, basketball, baseball, soccer, xc, track and field, volleyball, field hockey, lacrosse, softball, golf, tennis, crew and ice hockey. The skilled athletes go off in different directions.
Secondly, with a general lack of depth in distance running, American runners can still be successful on the track for a prolonged career. Ritzenheim and Tegenkamp have been on the elite level nationally for close to a decade. If one of them loses a few seconds in the 5k or 10k due to age, they are still among the American elite. There is minimal incentive to go to the marathon unless they really want to be on the roads. Conversely, there are so many sub-27:00 Ethiopian and Kenyan runners, the financial incentives are for them to get to the roads to have more opportunity to make money. If they lose any speed on the track due to age, they are an afterthought. I also think that American shoe sponsorship favors track performances over road performances.
I would also think that being young and having the ability to recruit different muscle fiber groupings would be advantageous to a marathoner. Because of the depth, many Kenyans are going to 1/2 marathons and marathons when they are in their early-20s. They are at peak physical strength and have not lost any fast-twitch muscle fibers due to age. Americans are waiting until their late-20s, early 30s.