For a little more context, there were Apollo flights planned out to 20, Nixon cut those. The Apollo Applications Program aimed at building a lunar base and observatories up through '76 but the Johnson administration declined to fund it partially to fund of Great Society programs but also partially because no one could adequately explain what a lunar base would be worth. We did get Skylab and the Apollo-Soyuz project, so it's not like project ground entirely down to a halt but what we did eventually get was a far cry from the heady dreams of the mid-60s, that's for sure. And, as cool a vehicle as the Shuttle was the STS did not at all deliver on its promises, further sapping resources.
Why haven't we gone back to the moon? Funding dried up real quick once it was clear the Space Race could be won with the existing amount of money spent, new projects had to answer "why" in a concrete way to compete for limited resources and it was not at all clear what we'd go to the moon for compared to, say, building the ISS.
Space theorists in the 30s - 50s assumed that you would have to build up on-orbit Earth infrastructure before the moon could be sustainably reached, meaning you could go back whenever just because. That is, you need stable communication, transit hubs, refueling, space-based construction went the thinking. Both the US and the Soviets took big, expensive shortcuts in the Space Race but I find it hard to believe that the original analysis -- space is only as reachable as its infrastructure allows -- hasn't been showed true in the last 50 years.
And incredibly far, much further than I used to think - you can fit all planets in the solar system in the average distance between the earth and the moon!
