I think that there's, in general, a lack of discussion because Rollplay shows don't really promote the kind of discussions you find from TV shows where people talk about theories and callbacks to previous seasons/episodes due to writing/cinematography. The best threads are the threads immediately after an episode, but even then most of the replies are "fantastic show guys!" which is great but doesn't exactly spark any actual conversations. I also think that people replace talking about stuff here with talking about it in chat when it's live.
I think what Lief said above is pretty on the point. If there was actual stuff here from the creators/cast then it would draw people in more, but you can't expect your cast to be using/monitoring the forum when you've also got them on Patreon. And you obviously want people talking more amongst each other without needing cast/post-show threads. Which brings it to another point which is splitting the userbase. When you announced the move from Reddit many replies were about splitting the userbase and now that's split again by Patreon (people talking about Patreon-only stuff here means a lot of people not being able to chat about it if you're NOT a patreon). There's discussion happening but it's split between live chat, reddit, here on the forum, Discord and Patreon comments so it doesn't really have a place to "build up".
I also think forums are sort of a dying art. People tend to go to Twitter or Reddit for discussion/posting about anything nowadays so coming to this site is very different and not as accessible/pleasant to view. Replying to a user here puts it in the same spot as a reply to the OP post which makes tracking individual conversations difficult.
For non-show discussion, I think it's going to be hard to get people to talk about say, video games, when they can go to /r/games and get a more accessible experience. I think the main thing to remember is this forum is focused on you and your shows so that should be the main draw, so what can you do to spark more discussion on the shows themselves?
I kinda just puked my own thoughts on it out there but that's why I think there's lack of participation on this site. It's hard to nail down why but accessibility is definitely a huge one.