My guess is that a lot of small, tightly-knit (i.e. everybody knows everybody already) servers don't need or want all of the extra "community" stuff. Community features seem like they're mostly for managing loosely-knit communities that are big enough to need different trust levels, onboarding procedures, announcements etc.

In fact, the Discord UI says as much: "Don't [convert your server to Community] if your server is just for you and a few friends. Community servers are for admins who are building larger spaces where people with shared interests can come together."