PC gaming was, back then, still very DOS-centric. It wasn’t until the late 90s that games started to target Windows. And even then, people still had older games they wanted supported.
I'd say both. IIRC the DOS story was better under OS/2 than NT, but the RAM requirements were higher (at least until XP).
To add a third prong: hardware support was a big issue too as it is for any consumer OS, with legacy hardware being an issue just as it can be today if not more so. This hit both NT and OS/2 similarly.
NT 4 had NTVDM and it worked well enough. Quake 1, command and conquer, sim games for DOS and a bunch of other stuff worked just fine. You'd run into issues with timing on crappier games or some games that talked directly to soundcards I forget what the details were but you'd just not have audio.
I had a gravis ultrasound at the time and remember having to cut the reset signal line on the card.
I could then initialise the card in DOS and reboot into NT without it being reset and losing settings. Then some sketchy modified driver was able to use it.
I remember NT4 having problems with games that wanted to access SVGA resolutions and SoundBlaster. I kept a volume with Win98 back then specifically for the games.
PC gaming was, back then, still very DOS-centric. It wasn’t until the late 90s that games started to target Windows. And even then, people still had older games they wanted supported.