Becoming rock hard at night entails less need for grooming.  Certainly riders would need to adjust to their much larger increase in speed.  Its really just a safety issue as to why they don't do it.  Increase in speed, rock hard landings (especially if you short/overshoot), lighting issues, and maintaining composure in a poorly lit (compared to day time) full course.  A lot of slope courses don't give much spacing between the jump lines which would end up disastrous if a rider lands off balance and is pushed to a side, and then tries to regain composure and hit the next feature.   Its a combo of everything, maintenance being of slightly lessor concern though.