So, out of the 24 men that went to Moon orbit and back during the Apollo program, just three of them did the trip twice, but landings where always done by a different crew.
From the politician's point of view, it stands to reason that sending a fresh crew each time would provide the US with a bigger pool of heroes to show around. It would also add some variety to something that was old news very quickly. I can imagine people watching TV and commenting "There's that Buzz guy jumping in the Moon again".
From the point of view of crew management, it also makes sense to have a fresh crew on each flight, both from the point of view of the astronauts that miss their chance to flight (Hey, Buzz's been up there three times already, why cant I go at least once?) and from the poor guy that's sent once and again to the same barren wasteland.
But, from a mere Space Agency point of view, is there anything to be gained from sending a new crew each time (perhaps training, or researching how a new team will do things differently). It would have made sense if they sent scientist specialst before Schmitt went on Apollo 17, but the other guys where all pilots.