The U.S. is the richest country on Earth and was determined to win the space race for political and propaganda purposes. Because they lost the race to put a man in space and to orbit the Earth, they kept moving the goal posts until they won. But it was an enormous expense. Once the Soviet Union decided not to go to the Moon, for a long time there wasn't much reason or money for anyone else to try to be the second country to the Moon.

Even the U.S. soon stopped sending astronauts to the Moon, and there's still a lot of debate about whether they should be going back to the Moon again. Unmanned missions to the Moon or Mars or other objects in space are more practical for scientific purposes. Sending humans to a space station is more practical if you just want to keep learning how to keep humans in space.

One very big reason the U.S. is going back to the Moon now is a renewed space race with China, which recently sent three robotic missions to the Moon and announced intentions to put a base on the Moon by 2030. So although there are various other justifications, arguably it's really about politics and propaganda again.