Hopefully they'll send more drones to mars, moon, etc and less rovers. Unless there are technical reasons I'm not aware of, why wouldn't they scout with drones instead of rovers?
The science payload that an aircraft can carry is much smaller compared to a rover. Also due mass restrictions (afaik) copters are solar-powered, while rovers can be rtg powered; this is pretty relevant for cave exploration as it would be pretty catastrophic for a copter to run out of power in a cave where as rover could at least hypothetically find its way back out.
Perhaps because dust storms would ruin the rotors?
How would you fly in such a low atmosphere?
Same as on Earth, the aerodynamics are just a bit different. There's already a helicopter flying around on Mars: