Are you going to cite "Letsrun.com message board" in your paper? If so, please video the reaction of your teacher when he/she sees it.
But for fun, it is a combination of genetics (and I use that term loosely here), socioeconomic factors, and environment .
First, keep in mind the great runners overwhelmingly come from one tribe.
Genetics: David Epstein does the best job both in The Sports Gene and in his TED Talk. It helps as a distance runner to be light and small to dissipate the heat. Kenyans (and others) were well-suited since to survive and thrive in that climate it is better to be longish and thin than short and squat. (Epstein points out that as you move away from the equator native peoples tend to get shorter and more squat.)
Environment: I mean not only the climate but the culture here. The climate makes it possible to be active year round especially in the Rift Valley. Altitude maybe a help too. Also activity levels traditionally have been higher. While it might be overstated now the amount the walking done by kids in Kenya still probably surpasses what even 1% of US kids do. So that walking to and from school might be a factor. Not to mention if they are working after school being active every day.
Socioeconomic: I believe soccer is the most popular sport in Kenya (as in most of the world), but running offers a way out of poverty. In the US other sports are seen as a way out of poverty (although it is interesting to note that the NBA was once the cornerstone but now NBA players are coming much more from the middle class). People see a Kenyan win a marathon and collect $100K in a place where the avg yearly income is $1200 and there is incentive to give it a try.
My take is that in the US where the avg income is around $57K, the incentive to run to get out of poverty is just not there. Go to school, get an education and you will be ok.
The "heroes" do play a role. A colleague at the USOC likes to say that a sport needs a hero to build on. Cycling had Lance Armstrong (cycling membership exploded under him as did the performance of US riders). Skiing, gymnastics and other Olympic sports have "heroes" (or "sheroes") that they promote for the next generation.
Also, I do not want to forget hard work. They put in the training. I imagine running 2-3x/day and then chilling the rest of the day is a superior life to being a farmer. They also have figured out the training that works for them. I think the training groups are a very important feature (and one factor that has led to the US success is the growth of training groups).
I would caution anyone from thinking it is just one thing (even doping) that has made them successful.