I think what a lot of people prefer about the forums is that you can check in every now and then and follow along to the 10-20 people posting. And the thread topics are (supposed to be) more direct. My reason for leaving prior, is because I prefer smaller groups. I have plenty of group chats with HD friends, long before discord. And I love it. I don't feel the need always get feedback from 100s of coaches.
The discord is so large, that if you post in the morning, and come back at night (say after work), there could be 100s of posts since your last post on a busy day. So it can be overwhelming for some. I love to chat and be social. But sometimes, I'm ok with hearing the thoughts and comments from 5 people, rather than 100 people.
All that said, I do feel discord is good for the game. And you can sift thru whatever you choose to, as you mentioned. But sometimes a coach may not mesh well with another coach. There's some guys here in the forums that I love to banter and argue with. There's posts here that I love to read and learn from. There's some guys that post that don't post things that I'm interested in reading (not that there's anything wrong with that coach, maybe we just don't cross paths in HD ever, and don't talk about similar things). So to multiply this times a LOT (the number of humans in the discord), i can see easily how it can be overwhelming for a coach with a certain personality.
We're all different characters here. As long as we have forums, sitemail, discord, conference chats, off site chats, or any other way to communicate that I left off, I think everyone can find what works for them and be happy playing the game we love.