[nuccy]

Another non-obvious problem is that RTGs, as any other thermal machines, need a gradient of temperature to work, i.e. to generate electrical power there should be hot (nuclear material) and cold (radiators) side. On interplanetary spacecraft (Voyager, New horizons) Sun is in a predictable (and stable) direction so RTG's radiators can be put in a permanent shadow of the spacecraft. On the Moon the sun is moving, and there is no atmosphere (unlike on Mars where RTGs are used), so on a small spacecraft RTG will need to be dug deep into the regolith which is absoluteky non-trivial since just landing straight sometimes is a problem.

There are always tradeoffs, it is almost never "why don't they just" case in spacecraft development.