[nuccy]

Generally I agree, but Moon is not a bad place for solar panels if a spacecraft has no contingencies and is able to harvest energy during Moon's day and store it in batteries to be used over the night. The sufficient power can be generated by a solar panel of the size (or even smaller) of the spacecraft itself. The other story is for missions like Juno [1] or Europa Clipper [2] which use solar panels near Jupiter - instead of centering develoment and mass budget around payload most of the spacecraft is an enourmously sized solar array. Juno panels generate 14kW on Earth orbit and only 500W near Jupiter [1].

1. https://www.jpl.nasa.gov/news/nasas-juno-spacecraft-breaks-s...

2. https://www.nasa.gov/missions/europa-clipper/nasas-europa-cl...

[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.