There’ plenty of reasons, most of which have to do with the human psyche and error. I imagine it’s largely due to convenience. And then one may rationalize that initial thought by assuming that most of their potential audience uses Discord anyway, so they won’t consider other options due to just how damn easy to setup and monitor their community via a Discord-like app is. They may not consider searchability, or information access at all. They may give very little weight to the fact that their entire potential community is subject to Discord’s whims. They simply may not be aware of how beneficial other options are.

Humans do not act based on reason. They act on a mixture of emotion and intuition, and only reinforce their initial position with reason, of one form on another. There is no point of attempting to apply logic to why the people (generically) do anything because of that. On the other hand, attempting to look at this scenario from why something should be done a certain way, as opposed to why it is done a certain way, has merit, as it allows us to influence a decision before it is made in the instant it is conceived.

I don’t think participation is the problem. If you think about it, you wouldn’t want just anyone to post something on a platform without first engaging in said platform. That can only have a neutral or negative effect. People asking stupid questions or people cursing out users. The act of signup ensures that the would-be poster has to signup first and rationalize their post during that process.

Therefor, the problem must be something else, it is the information gateoff (amongst other things) that makes Discord and similar apps unfavorable for community management and information distribution.