I think it's all of the above. But if I can blame one thing it's the rise of the internet as THE place for fandom and the deemphasis on real life meetups and cons. Used to be the internet was the place to find out about and organize your real life stuff, and where you hung out between that kind of thing. Now it seems the internet is the end in itself.
I also think that everything moving SO fast means more shows get these fast, temporary fandoms, that sort of fizzle out. So people wind up obsessing over one thing for a few months, but if the show gets cancelled or ends, or even in hiatus between seasons, the fandom for it dies. The last show I saw that really got a new FANDOM that has some staying power was Good Omens, which had enough interest it got a second season it wasn't intended to have. But now that it's over, and ended...poorly, along with the hatred for Neil gaiman, we'll see if it continues to exist. In the before times you had huge, mega fandoms that people were in for YEARS. Sometimes you had people who were like...second generation whovians or whatever.
And of course there's the rise of "experience" culture. Subculture in general is not longer only there for those who look for it. I'm not saying it was HARD to find (well at least not in NYC) but you had to put some work in on facebook or Tumblr or before then meetup.com to find an event. Once you DID you probably met people or got some info that would lead you to the next one, or a group where things were organized. I was in so many Facebook groups for different fandoms and interests, and PEOPLE, people you knew, not algorithms, would talk about other events they were going to. And this was true for all sorts of subculture, historic costuming, burlesque , night life in general, they were all something you had to be in the know about to know about. I never really liked burlesque much, but burlesque preformers used to be rare enough that seeing one was a treat, and if you were in any kind of nightlife scene, you would start to recognize certain names, as there were only so many. Now, every event either has drag or burlesque, weather it needs it or not, and you never see the same person twice. There's just too much going on. And of course, that leads to the quality of all these acts dropping, and arguably, a watering down of what both these words even MEAN.
And I think it all comes down to the algorithm. Now that there aren't really social NETWORKS where you see your friends posts and what they're doing, events can't garuntee on that kind of word of mouth advertising, and with putting on events SO much more expensive, everyone's got to court the algorithm. And then the algorithm can show ANYONE ANYTHING. And then normies who just kind of see "omg a cool thing" because they're used to seeing these aesthetic tik tok videos about being subculture, decide to try it. But they're not really INTO it. But they sure do spend money because for them it's a "once in a lifetime" thing, not a culture and a community they want to be a part of. But then the company sees they're selling tickets and getting more money so they continue to court these people. And the same ones might not come back but there's always more. And the cycle continues. And I then think the anxiety of making social connections and talking to strangers kicks in and the people who want to be part of the community, don't make those connections, and with social media, again, more focused on algorithm than connection, you don't KEEP the connections and get to know people,so the community doesn't really...gel.
It's sad. I've seen this happen to Ren faire really badly. I'm watching it happen to the anne rice vampire ball in the three years ive been going. I feel like I went to the last good year. I miss the way fandom was in the 2010s. It was my home. And I'm so glad I've kept the friends I made in the old days. And I really feel bad for people who don't have what I have. It must feel very empty.