6 "Moon Landing Missions", 18 Different Astronauts!

There were no repeat "moon landing" missions for any of these 18 astronauts.

Doesn't that seem extremely odd, considering how tricky these missions were (in principle)? Wouldn't it make sense for at least one experienced astronaut to go back on a second trip?

It wasn't a matter of being exposed to radiation, since some of the "moon landing" astronauts went on other space flights that didn't land on the moon.