To be fair, there are real world safety considerations when putting radioactive material into orbit. It's not just a knee jerk reaction, in all likelihood. The risk for Cassini was calculated at 1:1400 (For context, humans have launched something like 8000 man made objects into orbit). That's certainly acceptable when there's no other reasonable way to provide the power that the mission profile requires, but it makes sense that they would go with a more conventional source where possible.