Discord has a really low barrier to entry for conversation, it's low friction. If someone playing a game gets a sorta funny screenshot they can share it in Discord and have a decent chance to get some responses or reactions.

But they won't post it on Reddit or anything because there's more friction there, it feels like more work to create a post and then you're unlikely to get any responses anyways, and even if you get upvotes you can't see who upvoted like if it's someone you know, and there's no easy emoji reactions. And users making comments to respond also feels higher friction than a message, people are just less likely to do it for whatever reason.

It just feels like small scale conversations are more likely to happen in Discord than anywhere else. If you have a small group of people then something like Reddit/Lemmy probably won't be sustainable, it'll never take off, but get them in Discord and you're more likely to keep the group active.