moon, but, there are much more obvious reasons, mostly money -
https://www.businessinsider.com/moon-missions-why-astronauts-have-not-returned-2018-7
The Chinese and Russians are working on a joint moonbase, so either the aliens have said it's cool now, or they left, or they were never really there.
https://www.space.com/china-russia-international-lunar-research-station