The *real* real answer as someone who joined the fandom in 2016? It was the devs.
There's a lot of completed fic out there, it's just usually a few years old at this point, and there's a lot of fanart, it's just also older. A lot of former swtor blogs (like myself) moved on to other games over the years after swtor kept getting worse. It got SO bad that the game was recently taken away from Bioware and given to another studio. THAT bad.
There were a number of contributing dev-factors to many in the fandom packing their bags and leaving. I don't know where you are in the story, so I don't want to spoil things for you, but basically all the story content after KotFE was... divisive, to say the least. It was pretty clear that the devs didn't give a damn about the years of story they'd built anymore, and for players that did care about companion characters, they were given pretty insulting narrative endings for almost all of them, drawn out over the course of irl years. For those playing non-force classes, it also felt as though the story was suddenly completely irrelevant to them - If you sign up to play Boba Fett, you're gonna be confused and probably disappointed when the story suddenly makes you be Luke Skywalker.
It looks like people have mentioned this in the notes, so I won't go too into detail on it, but the harassment. Oh my GOD the harassment. Fleet chat became a nightmare of slurs and bigotry, and it wasn't always like that, it was, like a lot of what the devs did, a slow frog boil.
Then there's the stuff that actually made people keep playing the game, the mechanical stuff that the devs started letting go. First, the economy became a nightmare. FtP players were capped at a ridiculously low amount of credits and prices skyrocketed to the point where if you weren't actively money farming as a subscriber, the GTN was unusable. Abilities that were rp-focused or immersed you in the class started to be removed - Infamously, people were really sad to see Sith Marauders lose the force choke ability. The release of 7.0 honestly broke the combat system and so for many removed the reason they'd log in every week. For anyone who was into the raiding scene, your raid team was basically thrown back to beginner level by how badly the new combat worked. The weekly rewards system being so limited (idk what it's like now, but it was really bad for the first year) made players feel as though they were being shoved towards content they didn't want to play, like galactic starfighter.
Fandom-specifically, there was kind of all the usual stuff you see on tumblr - the bad actors, the toxicity/harassment, the ship wars. It wasn't really that much worse than what you see in any other fandom, but the small size of the fandom made it stand out for some people, and there were some popular creators/writers who ended up just sort of throwing in the towel, and all of it was of course happening at the same time as the devs were torching their own game. I know for me personally, once I finished my longfic in 2022, I did it with kind of the sense that it might be the last thing I did in the fandom. I'd finished my passion project and was finding, as a lot of fandoms do, that creativity and passion are really difficult to sustain when the source material is actively being ruined
It'd be nice to see some form of the fandom return, but yeah. It was here, it wasn't huge but it was decently sized, and the devs didn't care
