Pi 4's were hard to get there for a while. Pi 5's are expensive. Lot of other SBCs are also expensive, as in not all that much cheaper than a 2-3 generations old low-end x86. That makes them less attractive for special purpose computing, especially among people who have a lot of old hardware lying around.

Any desktop from the last decade can easily host multiple single-household computer services, and it's easier to maintain just one box than a half dozen SBCs, with a half dozen power supplies, a half dozen network connections, etc. Selfhosters often have a 'real' computer running 24/7 for video transcoding or something, so hosting a bunch of minimal-use services on it doesn't even increase the electric bill.

For me, the most interesting aspect of those SBCs was GPIO and access to raw sensor data. In the last few years, 'smart home' technology seems to have really exploded, to where many of the sensors I was interested in 10 years ago are now available with zigbee, bluetooth or even wifi connectivity, so you don't need that GPIO anymore. There are still some specific control applications where, for me, Pi's make sense, but I'm more likely to migrate towards Pi-0 than Pi-5.

SBCs were also an attractive solution for media/home theater displays, as clients for plex/jellyfin/mythtv servers, but modern smart-TVs seem mostly to have built-in clients for most of those. Personally, I'm still happy with kodi running on a pi-4 and a 15 year old dumb TV.