Why haven't we seen forums move to Lemmy despite the availability of a forum-like UI?

I recently came across LemmyBB, a frontend for Lemmy that is based on the popular forum software phpBB. It seems like a great way to bring the familiar forum experience to the federated world of Lemmy. However, I haven’t seen any significant migration of forums to Lemmy using this UI. What could be the reasons behind this? Are...