I'm gonna say I wear tight/slim/skinny (whatever you wanna call it) pants because its close to what I wear around regularly chillin/around town-wise, and when I wear xl pants I feel like I'm about to go sky diving/wearing garbage bags as pant legs or something along those lines, or maybe impersonating mc hammer. In plain honesty though I like the look of skinny pants. I also don't ski and am more skate oriented when it comes to snowboarding I guess.
But in the end each to their own, no one needs to understand why others wear what they do.
I don't go runnin around philosophizing over why goths dress the way they do, its weird but hey they just do what they do yadadmean? just like us snow/ski folk.