While I'd love to see nuclear powered Mars rovers, we may have to wait until the next generation of RTGs is ready. Current RTGs are pretty big, heavy, and very expensive compared to solar cells and batteries for the same amount of electricity produced. The extra weight for the electricity source means something else has to give. Either you cut other things like sensors in order to keep the rover weight the same or let the rover get heavier but then possibly end up needing a more powerful (and expensive) launch vehicle.

Thermocouple RTGs are simple and reliable with no moving parts, but they're inefficient at converting heat to electricity. That inefficiency means you need more radioactive material, typically plutonium, which really drives up the cost. One of the companies working on the next generation of RTGs is looking at using either a turbine or perhaps a Sterling cycle engine to generate power. Such a system could be much more efficient than a thermocouple system, meaning you'd need a lot less plutonium and could possibly end up with a much lighter RTG. However, you'd have moving parts that can break and you might introduct vibrations that could disrupt scientific instruments.

RTGs can result in significantly higher costs in another way: due in large part to the Luddites' irrational fear of anything radioactive, putting RTGs into a Mars probe would probably cause a lot of extra costs for things like environmental impact statements, public hearings, and the like.

No one ever said it would be easy.