I found a paper on it:
https://bjsm.bmj.com/content/34/5/391
East African running dominance: what is behind it?
Introduction
East African middle and long distance runners are currently the dominant force in athletics. As well as dominating the track events at the last several Olympic Games, they are also dominant on the American and European road racing circuit and world cross country events. Although many physiological and anatomical factors have been proposed to explain East African dominance, research into these variables has not yet revealed any definitive advantage for the African. Traditional social and cultural factors have often been described as “advantageous”, and, although these factors may be to a greater or lesser extent involved in the East African dominance, it is probable that both the African and caucasian psychology or “mindset” are now additional important factors in maintaining that dominance. Like Scandinavian distance runners in the early 20th century, who won 28 of 36 possible Olympic medals over 5000 and 10 000 m, the East Africans have developed an aura of invincibility, both in their own minds and the minds of their caucasian opponents. Caucasians world wide are searching for proof of the physical advantage of the East Africans while handing them on a platter a psychological advantage which, until removed, will perpetuate the current state.
Possible reasons for East African dominance
Many factors have been described to explain both the dominance of East African middle and long distance runners and West African sprinters. Many of these have been disproved many times but are so fixed in folklore that they continue to be touted as possible causes of success. Despite being shown to be unsubstantiated in the 1930s,
1 Sir Roger Bannister, in a speech delivered to the British Association for the Advancement of Science in 1995, suggested that factors such as heel bone length, subcutaneous fat, and differences in Achilles tendon length may explain the advantages that West African athletes appear to have.
1,2 Proposed factors involved in the success of East African athletes include
environmental, genetic endowment, and
social development,
3 while the psychological make up of the East African is rarely considered.