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.
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.