Poser
WKR
I think it partly comes from the climbing/mountaineering world that these hunting lines emulated in the first place, where slimmer fits do make sense. Baggy legs get caught on crampons, and bigger clothing just weighs more and takes longer to dry. Slimmer fit pants work better for alpine climbing, ice, skiing, etc. Modern fabrics like softshells, lycra, etc, have also played a role in allowing slimmer fits that you just couldn't do in older materials.
I'm sure fashion has played a partial role too, just look at men's suits now compared to the 90s.
I think the biggest factor in slimmer fit is, once Sitka first hit the market as the OG technical hunting apparel, the push was for the athletic aesthetic as a way to further separate the brand identity from the likes of Mossy Oak, Scent Blocker and virtually every hunting brand on the market that was geared exclusively towards Eastern whitetail hunting . We even saw Under Armor jump in the game for awhile as they saw the opportunity with the athletic aesthetic.
I personally prefer a more articulated fit vs baggy/boxy, but the 90s are definitely back in resort ski/snowboard fashion -huge hoodies/jackets, oversized pants etc. Though, the backcountry specific apparel still leans towards a more articulated fit. If nothing else, I don't want all that extra fabric when I'm trying to fit it in my pack.