I feel like another good point is that discord servers are generally very easy and low-rent to set up, compared to setting up and properly moderating a technical forum where everything is supposed to be well-organized. Lots of smaller open source projects would have to take away time they'd actually use to develop their tools, in order to set up a forum and keep it running. In those cases, they're better off just using a discord server, and then hosting a quickstart guide or a commonly asked questions thing, and you can put either of those basically anywhere.