Mostly because there's been very little US activity on the Moon to justify it. Orbiting the Moon can also be a pain - its gravitational field is "lumpy" - but you can manage that by making your orbit bigger (higher). See this paper if you're interested in details as they pertain to the Lunar Reconnaissance Orbiter, which has been flying since 2009: https://ntrs.nasa.gov/api/citations/20070035736/downloads/20...

China has a lunar comm relay IIRC, to support some surface operations on the far side.