It hasn't. There have been unmanned probes ever since.
If you mean manned missions, there just isn't enough reason for them to justify the enormous cost. Why send people there when you can do just as much science with an unmanned spacecraft at a far lower price? A robot probe doesn't need air to breathe, food, exercise or sleep. So why would you do it unless there was something only real people could do?
Let's face it, the only reason the USA did it was from feeling hurt that the Soviet Union got into space first and it wanted to be first at something. So JFK set the target of going to the Moon. Which it did on time, great stuff, OK mighty America can do it, I'm first in line to say well done, and it was done several times to prove it. Now what?
Why would you build the biggest rocket in the world (which is what it needed) just to do that again? NASA is only doing a more modern version now with the intention of moving on to Mars.
Just out of interest, the Soviet Union never managed it. Their Moon rocket was too unstable and blew up every time they tried to launch it. Once it destroyed the entire launch pad and secretive as they were, even they couldn't stop that being picked up on seismometers as the world's biggest non-nuclear explosion. So the USA would have won the race anyway, but nobody knew that at the time.