It's a very different format compared to discord, and has it's place alongside other communication platforms I think. Discord is much more fast paced, and catching up can feel like a daunting task sometimes if you're offline for a few hours. In the forums I can catch up on specific threads quite easily, without having to dig through pages and pages of people chatting. Want to find the finished OC dog pile image? Easy. Want to see what people say about it? It's all right there. Conversations like that just don't work the same in Discord.
I also feel like it's a lot easier to get your thoughts out on a forum, because I don't have to worry about 20 other users chatting over the top of me and burying the topic I'm responding to. I know Discord has threads and replies, but lets be honest, people use Discord differently than a forum like this. If I respond to someone's post from yesterday that's 200 posts up in the chat log, I'm not going to get to have that conversation with them like I could here, I'd just be ignored.
Promotion is something I've heard people complain about a lot here, but tbh it's netted me lots of readers. It just doesn't work the same way on Discord.