Tech geeks and nerds (no offense, I'm one too) tend to be the first people to populate any sort of new online social network. Just the way of the internet.
While I do like Linux and talk about it pretty often on the fediverse, I do realize that 96% of internet users don't care about it and the lingo is...incomprehensible for most people. Even I get kind of sick of talking about Linux on here sometimes lol but unfortunately many of the things I wish I could build a community around simply don't have the fanbase needed on the fediverse to begin a community for the moment. While not impossible, building a community for relatively niche subjects on a small platform like this that is in direct competition with sites like Reddit is very difficult and easier said than done.
My advice is just try to search around and find things as close to your interests that have active people as possible. Looking for broad communities can help out here, for instance, just go to /c/art of whatever instance instead of trying to find a /c/painting. Also, for Mastodon especially, use hashtags if you haven't begun to already. Mastodon was wack until I started using hashtags extensively, they somewhat make up for the lack of a recommendation algorithm.
Its pretty rough around here if your interests aren't related to tech/FOSS/linux, but that should make things a little better. Hopefully there will be more diversity in subjects on the fediverse in the future.