It such a combination of things that I don't think any one thing gets the blame....
EQ was set up perfect in terms of timing. It was presented to an audience who had only dreamed about something like this over the years. Gaming was much more of a family/friend affair too. We didn't have all of the meaningless connectivity that people do today. Relationships mean next to nothing to them in the online world. The people we were exposed to were very local outside of some online interactions (Diablo, Unreal, Starcraft, UO, etc.) so it was very limited and in its infancy. The gaming generation was at a fairly mature age too compared to now. Whether it was the Nintendo generation or Table top, those people were anywhere from 16 - 26.. some even older. People playing EQ probably played Sierra PC games, Zelda, and Diablo. I say all of that to really point to the comment about life long friends. EQ was bringing a network of people who played games to enjoy time with friends and what would even become families for many of us. Gaming today is very rapid fire and serves an audience so broad in age in purpose, the formula simply does not work the same because nothing in its environment is the same.
It's unfortunate because the key ingredient doesn't exist. The time and the people have moved on with the world. That doesn't mean its hopeless but AAA companies want the financial success that MMO's touted for years. They create systems that throw people together, progress through content, provided shortsighted rewards, and a very shallow world. EQ (and others) was a game that just found a lot of accidental success and we benefited from it.
The good thing is the gaming world, especially MMOs, have a decent opportunity because to captivate an audience who seem to like storytelling and online experiences again. Thankfully games like Fortnite and other MMO's have swept some of the annoyances away. So that's why I believe many are failing. They can't take advantage of audience that invaded our special little world any more because they're playing Fortnite, WoW, or whatever the flavor of the week is now. FF14, BG3, and other games have shown that we are still here and we've gathered a decent little following so if someone is bold enough to make a game that is the next evolution of an "EQ-esque" it would probably do very well. However, as long as people keep rolling out these shortsighted games, we're going to hate, the annoyances are going to try it for a week or two, the streamers will complain, and everyone will leave and wait for the next.
It's been kind of sucky to weather this lull but thankfully its been a cultural shift too. Stanger Things, D&D, and I'm sure others can point to specific pieces of media and culture have separated our worlds a little bit and it feels much like the setup we had in the 80's and 90's the created the gaming community EQ accidentally and luckily fell into. So here's hoping for a great MMO in the near future but in the meantime, I'm thankful for games like Tears of the Kingdom, BG3, etc. proving what makes great games and just imagine what would happen if we could put a few thousand people to experience those worlds.