The crux of it is that internet chat media are for chatting. You know, conversational back and forth between 2+ people who are actively responding to each other within that shared context, lasting for an extended number of back-and-forths. The less that a given type of infomonging can be described this way, the less appropriate internet chat media is for the job.
And that's it! That's all there is to why "join the discord server" is a joke we invoke to make our friends groan: a newsletter is not chatting. A wiki is not chatting. An email is not chatting. Absolutely none of these other media are bad; they are often quite good actually! But they are not chatting, and trying to run/distribute them via chatting makes everything worse for everyone (very much including the people making that Discord server instead of a newsletter/wiki/email/etc).
discord even had to go and break all external links to any media shared on discord because people were using it as a file hosting service. like, c'mon people