You want the truth?
1) They are so many. In the rift Valley regions , a lot of girls and boys gives a try to distance running, cos is the" easiest" way ( if you have medium / great talent ) to support your family. not everybody has the talent of Kipchoge or Brigid Kosgei, but also if you will be a 63 half marathoner or a 73 half marathoner thats enough to come Europe and to make a living out of it sending the money back to Kenya. In Europe is not like this . You can find in every town hundreds and hundreds of boys playng soccer but very very few runners.
2) in general their BMI is lower than a white caucasian runner. And this is a very good genetical advantage when it comes to distance running. Your power/weight ratio will be better.
3) Born and living at altitude, and the dedication they put in training , big training group, but these last two things can happen everywhere.
4) then the most controversial point, it seems then when it comes to doping their are second to none. The hundreds of runners suspended in last few years say everything no need to explain much more. When you catch people lile Rita Jeptoo (2 chicago and 3 boston then DSQ, Paris , Milan Lisboa, Stockolm) , jemima sumgong (Olympic gold and London marathon winner and Rotterdam) Asbel Kiprop (olympic gold and 3 times World Champion) and many many others.... you know that the problem is Huge......
So you put togheter the above 4 point and you have your answer. But considering the fact that in the rest of the world very few people try to make a career out of running . Their domination would be the same also in a world without doping,,,,,