I evaluated phpBB back when I took over the old Joomla site and was looking for a better option. The main reason we’re on bbPress is because it integrates very easily with WordPress (since it is, after all just a WordPress plugin). Using phpBB or some other system would require a lot more integration work that I don’t have the appetite for, particularly around the user interface and account management.
Keeping everything in WordPress makes the site maintainable and usable without needing a lot of my time to support the admin side. If that weren’t a consideration, we’d be running on Jekyll + Discourse, which I’ve used independently with great success in the past.