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.