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.
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.