Why do mogul skiers wear baggy clothes whereas downhill skiers wear Spandex?

It's all about aerodynamics. Downhill racers wear those ultra thin suits to maximize speed. There's no need to have patches on downhill racer's suits because the focus is on speed and overall form. You're right about the knee patches on the mogul skier's pants though. The focus for the judges is on the skier's knees and it helps them to see the action of the legs as they absorb the bumps. Obviously, speed isn't the sole reason why mogul skiers are being judged and the run is so short that skin tight suits would be pointless. Fashion is also a factor with why they wear baggy stuff, as well as comfort, but baggy clothes suit freestyle sports like mogul skiing.