From what other members have already said, It seems some of the biggest reasons why forum participation and engagement has been at a decline is:

- Evil fee for new users (my friends had this issue and they didn't push through with the registration as Bitcoin tx fees are high compared to few years ago.

- Merit/Rank up: I'm sure a lot of people would debate this but this has been a drawback especially for new users who are still learning how the forum works and working on improving post quality. It sucks to be at a position for a long, long time such that if you're not focused, you can just give up and try luck elsewhere.

- Old forum software: SMF software is one of the oldest out there and while I like the forum software when I joined in early, it's time to move on. There are better softwares around that could aid user engagement, rich user interface and also provide solid user experience especially in times like this where having good user experience is a huge plus. Forum needs to upgrade and fast.

- No hostile attitude go other members: A lot of people don't talk about this but if you're at a place where you're constantly getting the heat for participate in open discussions, then it'd be best to just leave the place all together. That said, I'm appealing to members to be more accommodating.