The reason is because people aren't creating those communities on here. If you want a community, the best step is to do it yourself, unfortunately.
You can try to talk to subreddit admin or mod team about it, but I think it's just that Lemmy needs more people to "do" than "want" if you understand what I'm saying.
Some mental health subs on here would be nice...
Personally, I'd appreciate more art subs being on here as well!