Another vote for genes. And that's not to say that other factors are not important. Of course they are. But one only need look at Kimeto run to see how incredibly efficient and built for running he is. Training helps to a point, but there is no amount of training that would get me to that build or that efficiency. Ryan Hall is the closest I've seen in a white person, which is obviously no coincidence. The outliers are going to be there, but if we quantified the desirable traits the mean values would be clearly shifted in favour of the east Africans.
Sure, Kenyans and Ethiopians are different genetically. We're talking about a very large number of genes, and there are many genetic routes to having traits that are desirable- just as between species there are different genetic routes to sprinting well, swimming well, or flying well.
Genetic non-believers have their heads in the sand. We're not saying genes account for 100% of the variance. We are saying that, on a populations level the field is not level, the same way it's not level for sprinting.