Renowned German sports journalist Hajo Seppelt claims he spent time in various parts of Kenya where he posed as an undercover sports agent and found out that doping is rife not only amongst Kenyan middle and long distance runners and but even with some of the foreign athletes who have been training in the famous high altitude areas of Iten.
— Athletics Kenya confirm use of banned substances

I guess it IS something about the region that produces some of the world's best endurance runners, pity it's more than just the altitude, hills and way of life and training for some of them.