Over the last 10 years the world has moved on in so many ways, and MMOs are simply unable to recapture the 'magic' that only those who played them during the glory years would be able describe.
Being in an MMOs used to feel like being part of 'the next big', (sad as it may sound) it was almost like leading a second life and because of this they really used to feel like something important or a big deal. They made you feel part of a community and connected to so many other in ways that had preciously not been possible. They made you really care about the success of your character and getting gear/boss kills or whatever used to feel like such a big deal.
15+ years later however and the world is now much more connected with the rise of social media etc. people no longer feel the draw of a 'second life' and as such apathy towards these games has increased massively.... Whereas once things like getting gear, taking down raid bosses etc. really used to matter, now people feel its just virtual pixels etc (which it of course is!). and in the grand scheme of things doesn't matter a jot... Expansions will come and go... gear will get replaced... its all just the same, rinse and repeat ad-nausea. Put simply it really does't matter anymore (some will argue it never did and they wouldn't be wrong).
As others have said MMORPGs are only going to decline further. Blizzard realised the world has changed which is why they stopped development on Titan.
Beyond that a lot of devs more recently just don't "get it" - trying to push certain gameplay styles or limitations in a game type where one of the big appeals is that you can create something unique to you to a certain degree and stuff like that.
Pushing style over substance game mechanics that are often non-intuitive, clunky or ultimately unrewarding.
Ignore their user base even when very pertinent issues are highlight and/or better solutions suggested - some of the older games were successful for a very long time because the devs didn't just take their long standing players for granted when big changes had to be made.