Eh, there's a lot of reasons, but to highlight a few:
-Lack of thargoid storyline development. The buildup to their reveal was great, it felt like a genuine mystery in the game. Things were good for a while but it relatively quickly felt nothing like an alien invasion and became a weekly server tick 'update'. No capital ships, no destroyed systems, no escalation (engineering AX weapons to take on more interceptors at once, etc). At this point, what is happening with the thargoids?
-In a similar tone to the thargoids, the guardians, Col 70 sector, Raxxla, and many other "things" hinted at through the now-defunct Galnet. These teases, much like the mystery of the thargoids early on, added depth and drive to explore and find out more. Cannon Science was great to follow in what they did, but now with none of that progressing, that hook that got me into the game is gone.
-Update bugs and what comes across as a developer that doesn't seem to listen to their player base as much as they should. Granted, they have been making improvements in this area, but I still feel there's a sense that they make their minds up and only after significant outrage by the community do they backtrack (though not all the way at times). This issue isn't a significant reason I stopped playing, but it has tainted my enthusiasm for the game.
On top of those items, I reached the 'end game' for me and own all the ships I'd want to own, done all the things I want to do. There's simply not much else that is really driving me to hop back into the game and continue, at least for now. I've found a handful of other games that are fresh and new to me, and with my personal time being important to me, I'd rather focus it on things I enjoy.
The cherry on top is that it seemed like these could have been "relatively" easy things to continue/fix, but it seemed that Fdev decided to instead, put all their eggs in one basket with the 'new era' coming up and drop the rest of it like a heap of moldy potatoes. It could simply be that timing was off and they didn't estimate on how long the new era was going to take to develop. It could be that many of the above items I listed will be in the new era, but the problem is that they couldn't stretch what they had out long enough to make it work for the longer-term players. I'll definitely look forward to what the new era holds, but I really hope that Fdev decides to start sharing some info soon because another 7+ months of nothing will likely result in another 7+ months of not playing.