Overwhelming weight. If you're trying to manage a community and you want volunteers, you want to minimize the "hassle-hurdle". You can either go entirely against the grain, or use something that everyone already has. And even if begrudgingly, everyone has discord.
I wish it wasn't proprietary products that won out in this regard, but they did, and there's little point shooting yourself in the foot on principle.
I wish it wasn't proprietary products that won out in this regard, but they did, and there's little point shooting yourself in the foot on principle.