It's specifically a forum with branching threads, popularity scoring, and topic-oriented communities that ordinary users can create and moderate. That's the successful formula.
Lemmy is missing that last feature. I hope that's merely because it's not implemented yet and not the result of an intentional design decision.
You can create communities on instances other than Beehaw. Beehaw has some restrictions which include not being able to downvote and not being able to make new communities, so yeah it's an intentional decision on their part.