[jordanb]

A good reason is the lack of availability of the needed isotope (Pu238).

The Europa Clipper has a huge array of solar panels instead of an RTG due to the last of the available supply going into the New Horizons spacecraft.

Pu238 was a cast-off isotope from nuclear weapons development so it was more readily available during the cold war. We should be happy that it's scarce now.

Also solar panels have gotten a lot better than they were when Voyager was launched, but even today anything going out past Saturn is not going to be able to use solar energy.

[wffurr]

There was just an article about a whole array of “nuclear batteries” using all sorts of decent chemistry: https://news.ycombinator.com/item?id=42118306

Are any of those candidates or are they just too small or with too poor a mass ratio compared to plutonium RTGs?

[jordanb]

Maybe. I don't know. I'd point out that tritium is a gas, so it may be hard to contain for long periods of time especially in space.