Because it is difficult to have a “chat like” discussion on chain sometimes. Also, discord allows radio style talk shows, so much of the conversation happens there.
There are many posts made on chain to have these discussions, unfortunately sometimes people don’t know they are happening or how to find them. Having one account where maybe a month or bi weekly “discussion” post is made might be a solution, I’m not sure if that would be helpful or not.
I do think it’s important to have the information on chain as well though. And look forward to a day when discord isn’t needed at all 🙂