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CyanicenineBetter freedom of movement as well as better coverage so you're less likely to get snow down your pants are my primary two reasons.
I'm not wearing 2XL or anything, but I like a baggier jacket and pants so that I can reach for a more difficult grab without tearing a seam. I've also experienced firsthand the draft that you get from having a jacket that is too short that exposes an inch wide strip of flesh when you bend over to grab a dropped pole, etc.
I also find I can't make full use of my pockets with more fitted pants, as my car keys are more apt to stab into my side, and my phone inside its waterproof case is more likely to bulge out like a malformed tumor. You can also fit body armor, like impact shorts, spine guard etc, if you chose to wear it. Baggy clothing is simply more functional and comfortable.