Is there a reason why projects opt for a Discord server rather than setting up a subreddit? Is it simply because Discord is easier to moderate?
With a project discord, you're filtering out people immediately -- typically only people who want to be there wind up there. It makes things easier for a lot of people this way.
Well. Yes. You filter out a lot of people, including legitimate users who would be interested but aren't going to deal with Discord. I'm not convinced that's actually a win.
Posts aging out mean that discussions fade from view. That means that they die off, as other people don't join in. Combine that with karma, and you get people tending to just contribute content that is counter to community building.
I've seen some similar stuff happen on Discord servers (I'm looking at you, Discord servers with 50 topic channels), but I've also seen people slowly get to know each other better there.
Or, to summarize: I generally see Reddit as better for discoverability and Discord as better for community. But they're both inadequate as a whole.