People will turn everything into a Discord server nowadays, no matter how bad of a match it is. I've even seen a Github project disabling their Issue tracker in favor of Discord, which is completely insane to me.
IRC is a completely open source protocol that anyone can develop clients and servers for, it's been around for decades and has an army of dedicated fans and devs.
Discord is a program owned by a private company that everyone is using to chat while the company slowly implements pricing and walls their proverbial garden off.
With your one Discord account you join all the servers you care about and interact with online/IRL friends.
There are downsides that people at Lemmy will find dealbreaking and I absolutely agree with them, but the convenience of it being a chat, forum (almost), voice/video calling and gaming platform for free with (currently) no ads triumphs all of that for the majority of people, and that's what makes it popular. It's also cross-platform so no one's left out.
I don't have proof of why discord is getting shoved down our throats everywhere but I think it is probably in large part because there is a fuckton of investor money behind trying to make discord swallow up every online community it can and they have kept costs to customers/shitty behavior dialed back until they establish an unassailable dominance over the market. Everyone just thinks it is great and it doesn't cost much money so what could possibly go wrong??
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.
I remember that post from MrGrimm. It went something like "there are no good image hosts for reddit, so I made one".
When they added user accounts a few years later to spin it off into its own thing, I was among the first to create one. Managed to snag the username "anonymous" before anyone else could. Never actually used the account until last June, when all the 3rd party app drama started on reddit. Now I equally split my time between here and Imgur to replace reddit: I go to imgur for the memes, and Lemmy for the news. Don't even miss reddit.