In short, this forum intrinsically promotes two main types of users:
1. People who have a question and leave when they have an answer.
2. Regular users who invest a lot of time to follow topics and conversations.
Becoming a casual, slightly regular user is hard because it is hard to find or follow the topics that interest you in this torrent of activity, in this huge archive. The site won't call you back with the new stuff that might interest you. This ends up driving away many users, and this probably reduces the range of diversity in terms of musical background and experience, age, gender, English fluency...
Even starting as a new user isn't very easy. Limited formatting, and people with Internet expectations of 2020 find hard to share their links in all their glory (e.g. embedding media hosted in Soundcloud / YouTube / Instagram...).
The motivations to stay are based on your time available to go through the list of new posts. No "likes" to recognize others' posts, no "likes" recognizing your posts. Leave alone badges, trust levels, incremental features as you gain experience, full fledged user pages...
Discourse also offers very sophisticated moderation options which, to put it short, end up contributing to a more friendly and welcoming forum for everyone, and not only for those who think that it is OK to be harsh and blunt, bite newcomers, and so on.
I understand well how complex and even scary it is to even think about migrating an existing forum to another platform. I have gone through more than one migration myself, and I see that many communities have moved from phpBB to Discourse in the past years. While I'm new to MPCs, online communities have been part of my work for a long time. If you want to discuss or even experiment, I am very happy to help, and I can even set up a test instance in some server.
Disclaimer: I have no affiliation with Discourse, neither I have an economic interest here. I am just passionate about online communities, I know and I love Discourse (just like I knew and I loved phpBB 20 years ago), and I really believe that these great MPC forums would get to another level with a piece of software up to the task.