Good magnification microscopes seem both light and cheap. As far as I could find out, there hasn't ever been one on any Mars rovers. The closest I could find is the Mars Hand Lens Imager (MAHLI) which only has a resolution of ~14 µm/pixel, closer to a hand lens (hence the name).
I can think of two reasons:
- There is a technical limitation: fragility, reliability, ...
- It isn't expected to teach us anything interesting.
Number 1. seems unlikely to me: there are plenty of cameras on the rovers, which don't seem fundamentally different from microscopes. Number 2. would also surprise me: there are a lot on earth you can learn from looking at rocks or soil under a microscope, why would it be different on Mars?
Since NASA scientists are actually pretty smart, I assume I'm missing something, so:
Why is there no microscope on the Mars 2020 rover?