A combination of feeling like we already beat the Russians and not really having something clear to gain by continuing to visit the moon. It's extremely costly to play golf on the moon.
There is probably a lot of truth to that. Certainly the political point was made, and that really didn't require follow-up reinforcement. Not only is it costly to play golf on the Moon, but the ROI for manned space exploration is significantly lower, generally speaking, than for unmanned exploration. There are obviously places and times where you want to be able to put people, but doing routine exploration in forbidding environments is usually better done with unmanned equipment. For example the data we've gotten about the heliopause from the Voyager craft would be impossible to get with a manned expedition using our current technology.
Honestly, if we really want to see whether it makes sense to get people out into space, let's open up space to private exploration and see what happens. Private enterprise, generally speaking, will only do something if the investment provides positive economic returns.