I think theirs a few reasons why people are leaving honestly.
1. Gameplay/job reasons, it's not a secret that people have been complaining a lot about watered down combat, healer & tank design are a great example why the current version of job design just doesn't work well, I'd like to say dawn trail at least has fun casual content, but to many the new raids savage versions were pretty lacking to many. I think one thing we're clearly missing is job variety, again imagine being a tank main for example, you should be able to pick up another tank and have a pretty different experience, but in reality every tank feels the same so theirs no replay value into leveling another tank job loses it's value a lot. I think honestly if they made jobs more fun at their core, we'd see a lot more people invested into learning different jobs which makes the battle content stand out more as replayable, but currently when everything is very samey theirs no point.
2. Content coming out is slow, patches taking 4 months (or usually longer), is not enough. waiting 4 months for what is a few weeks of content at most is a stretch, theirs also never any real exciting balance changes that keep things interesting to look forward to, so it's only usually the content itself which is mostly a alliance raid or 4 8man raids and a tiny bit of story with a trial if you're lucky.
3. going into too many "directions" this game focuses on Varity a lot, but apart of that deal is not everyone will like certain parts of the game, but this also means people who likes a certain part of the game would get less of that, then a game that mainly focused on one part, I do feel like MMO's do tend to want Varity, but waiting ages for small updates because of that isn't fun. I personally don't like alliance raids at all.
4. Open world doesn't really feel open, This is something common, the open world really doesn't feel like a place we even need to go to a lot outside doing the msq, you end up sitting in a city and queueing for the content you want, theirs nothing to really "explore" theirs no real challenges out in the open world.
5. Story related reasons. Don't need to really explain, personally i wasn't too bothered with story, i enjoy msq but it's nothing make or break for me personally.
With this most of the games appeal feels like it's towards the user generated side (aka role play, clubs ect.) which is the only real "endless" part about the game, I like this game but I have even not been in the mood to play it when theirs nothing to actually do.