This is very subjective, but I think Outlaw and Sin are more popular on forum in last couple of years than Sub.
I have no poll or other data. I can tell only for myself.
I think it is less popular because spec force you to be, let’s called it “pirate” fantasy, one fantasy. The previous version of the spec wasn’t just one theme. You could be assasin, ninja, swashbuckler, agile warrior. It was a simplier spec, focused only on hard hitting with swords/maces/axes, even daggers. That’s all the “magic” of Combat imo - without one pirate theme like today. (I play WoW since 2006 btw so I played both).
Well, and good for you. It’s awsome it makes you feel good.
Only if it’s bother you.
Ninja
Viable in PvE, single and multitarget, M+ - Sub was initialy rather PvP spec
Nasty PvP abilities, burst dmg master - the best of us could delete players nice and quick - for me, mostly M+ player novadays, it’s just potential to relax on random BG from time to time, ninja caping flags etc. - old times
Nostalgia - Once Sub could wear 1h weapons in both hands (it was so cool with gigantic fist weapons from Outland), was specialised also in bleeding abilities, tooked away eventually by Sin - I play Sub still hoped it regain something from its previous diverse talents.
I play all 3 specs and my choice first and foremost depends on how good they are in the content at hand. Subtlety was really solid for all types of content in the second half of Shadowlands and thats why I ended up playing that. Outlaw is now making a comeback, so as am I for the spec. Besides that of course I have my preferences, but they are secondary to the objective of clearing content.