Losing its hype. It's normal with every big thing out there.
I'll put myself as an example: even though Kancolle and GFL are my drugs right now, and WoWs, Hell Let Loose and 2 games are the only things I'm playing right now, I still play the games time by time, hear its music (two of my most listened artists in my Last.fm are Touhou doujin groups) and read its doujins and fanart posted here.
Hype is entertaining because you can easily found other people to talk about that hyped topic.
Another example: Age of Empires II has a extensive fanbase even though the game was launched 20 years ago, to the point of getting a remasterization and updates because the fan kept the game alive. And, damn, those are very dedicated fans. Yet, you don't hear about the game outside of official updates and the E3.
Comiket numbers are going down every year oh nooo (Please ignore that Reitaisai numbers go up every year)
Modern Reitaisai outperforms the number of circles that ever showed up at Comiket, even during its peak. While it doesn't get as many total visitors, nor does it have the total number of Doujin circles spread across the entirety of all media represented there, when it comes to Touhou and visitors specifically for it, it's absolutely enormous. If you're going by doujins that end up being scanned or translated or whatever - the majority of doujin work is fan fiction, first off, not doujinshi or fangames or music, and Reitaisai has a higher concentration of that - it's because less people travel to Japan specifically to check out Reitaisai to pick things up and scan them.