The forums have a different set of people and community than the discord- don't get me wrong, I still think both are cool places, but I've been here for like, half a decade or something.
The main draw to the forums is the accessibility to non-sharder users, I think (whether they be spambots or new users) compared to the discord, but I can't really speak for the discord.
There's also the community section of the forums which contains things found not on the discord. The Last Post Wins, TLT, and forum games really don't have any sort of parallel in the discord, and of course, everyone's favorite: the RP subforum.
Also the discord and the forums don't really seem to have too much overlap all things considered so stepping into the discord felt like entering a completely new space which was a bit off-putting. Why go to the discord if I just want to use the forums instead? I'm more familiar with the people on the forum and I'm not really seeking anything I can't find on the forum in the discord anyways. There's not really a significant bridge between the discord and the forum that makes it appealing to the long-time forumers.