I guess I had that one coming.
To keep it brief, difficulty curve (which isn't a big deal since every FG has difficult aspects), inaccessibility to compound the difficulty curve, and roster/moveset bloat.
A lot of it does have to do with people unwilling to learn for various reasons. Maybe I'm just baselessly going on about something that isn't for everyone anymore. :-/
While Tekken's popularity has certainly dwindled over time, it has less to do with Tekken and more to do with the changing environment.
1. Addition of online.......i.e. people find out that they suck. lol. This was me when I first went online in Tekken 6.
2. Genre relevance........FPS games become the new big thing. Fighting games become more niche.
3. Console relevance........Tekken used to be the reason to buy a console during the PS1 and PS2 days. But with Sony's dominance in the console marketplace completely fading away in the PS3 days, Tekken also no doubt took a hit. Namco clearly had to make the choice to make Tekken multiplatform with the market realities and Tekken no longer was that must-have exclusive.
Among many other reasons.................
Look at TTT1. That was A LOT of characters. It doesn't have to do with roster bloat.
The tag tournament series are not canonical and are meant to be a celebration of everything before. Yes, there are a lot of characters in TTT2 but the entire top row of the character select screen is likely not going to be getting an appearance in Tekken 7.