it's also the size of the userbase. there's something like 2k active users per month - when you're talking about niche fandoms, you might get 20 users per month, or worse 2 users per month, and people just give up on the comm without active work from the mods to cultivate a userbase. federation helps a little but ultimately, hexbear's politics makes things harder.
all that said, if you have a plan, make a post in /c/commrequest explaining your plan - admins will want a mod application to go along with it, mainly to make sure they're not modding a fascist or the like.
Hexbear is sorely lacking communities for every individual model of car, every Anglophone town with a population over 20k, and every esoteric combination of dietary restrictions (e.g. VeganKetoLowFODMAPIntermittentFastingGainIt)