The issue is that a lot of expert topics are stagnant. The community dies because the topic is already dead.

i.e. I follow woodworking but the communities are stale because there is rarely anything new. Woodworking is about old as a human topic can get. It’s just new memes now.

Hacker News is lively because the computer industry is brand spanking new plus HN also covers non-computer topics that are constantly changing.

I follow skateboarding and there’s always something new and always something to chat about.