> Were Martian environmental parameters less well-known (e.g., how gravity, air resistance / wind, and scree dynamics interact)? Bot performance in Martian environment (e.g., motor power output) less well-known? Were they being understandably conservative with their irreplaceable bot?

I haven't worked on the Mars rover programs, but I have worked on a NASA moon rover. Based on that experience I would hazard a guess that it's all three of these things.

Environment: We actually have quite a bit more experience roving on Mars so I expect the environmental uncertainty is a bit attenuated for the Mars missions, but one thing I was really surprised by during my time on the moon rover mission was just how little confidence our scientists had about the physical conditions at the specific landing site we are targeting (which is quite far from the Apollo missions for example)

Performance: Building accurate hardware tests on Earth for operations on another planetary body is incredibly hard. Of course NASA engineers are smart and can do this with fairly high confidence, but it's far from perfect. For the moon's 1/6th gravity, we couldn't just build a copy of the moon rover and test it on Earth because the suspension would collapse, so we had a whole separate copy of the moon rover mobility and suspension that was built to operate in Earth gravity yet mimic the performance the actual rover would have on the moon. That unit in and of itself is a feat of engineering that is definitely not immune to mistakes.