There is a dumb ass simple explanation: in about the same time it took to build rockets that took people to the Moon, we figured out that there was nothing to do there for humans. Or anywhere in the Solar System for that matter.

I mean until 1960 people expected to find whole civilisation or at least intelligent life on Venus and until 1964, at least some life on Mars. By then, it was too late to stop Apollo, but already no point to go anywhere else or expand Moon presence.