A couple of years ago, I was in Kenya (including Eldoret and the rift valley region) for a month. I noticed a combination of factors (that are typically mentioned when this discussion comes up), but when you see it for yourself in person, you quickly realize the difference between the way kids in Kenya grow up and the way kids in the US grow up.
Food. I average 50+mpw training for marathons, and I didn't run a single mile or hike a single mtn while in Kenya for a month. Yet, when I got home, I had lost 10 lbs (and I ate everything that was put in front of me). But it was mostly vegetables and fruit, and definitely no sodas, processed foods, or junk foods.
Exercise. Kenyans walk or bike everywhere (and often long distances). Salazar's comment above should not be dismissed too quickly. Few families have cars. Few families would spend their money to take a matatu anywhere. Kenyan's are constantly on their feet... walking, working, plowing fields, etc.
Altitude. Yes, US runners could train in the Rockies, but most Americans aren't born and raised in the Rockies. Virtually, all of the Kalenjin are born and raised at 6000+ of elevation.
Culture of running. Their sports heroes are their runners. I was there in April 2004 and the headlines of the East African Standard (the frontpage, not just the sports section) was the news of Kenyans sweeping Boston and London. Those are their heroes... unlike America.
Motivation to get ahead. There are so few avenues out of poverty in Kenya. Running is one of those for them. American kids don't have that extra motivation to get ahead in life by pursuing running.