The difference in marathon times between Ethiopians or Kenyans and people from other countries is very very small, in the order of ~2%. How do you know it's not cultural? For example, in running, no one thought you could run a four minute mile, then as soon as one person did thousands did as as well. It could easily be explained that too athletic talent is far more likely to go into marathon running instead of other disciplines in those sports, as is the case for regional dominance in many other sports, so in reality the actual advantage is even slimmer if it isn't null at all.
I'll end with a question - competitive cycling requires the same abilities as marathon running, that is, optimal oxygen intake and usage, and great endurance in the buttock, leg, and foot muscles. Why are Ethiopians not dominant there?
I am pretty sure the effort characteristic of endurance running and endurance cycling are very different. You use your legs for both, but the muscle groups are different and they way the muscles are used is different.
Also, to present a possible answer to your rhetorical question: cycling is a sport for rich people. A good bicycle, that a kid that wants to pursue the sport must get early in their life, is very expensive.
Not as much as you'd think, many of the muscles are in common when you use clipped pedals.
The most important factor in either sport in any case isn't the muscles, it's oxygen intake and use efficiency.
As far as your argument, by effect of selection, doesn't that meant that top runners are more likely to come from poorer countries? You also don't really need a good bicycle to train, just to compete, but even that's pretty expensive so I take your point.
You realize cycling is one of the sports that is most under scrutiny when it comes to doping, right? Unless the advancements in doping have produced a method that is undetectable chemically or through a rider's bio-passport, doping is much less of an issue now than it was any time in the past 5 decades.
I'll end with a question - competitive cycling requires the same abilities as marathon running, that is, optimal oxygen intake and usage, and great endurance in the buttock, leg, and foot muscles. Why are Ethiopians not dominant there?