But to be honest, before subreddits had Discords they had IRC channels. This has always been a thing.
They shouldn't also need to half heartedly try to compete with Discord. They should drill down on what they excel at.
Even though I am a daily Reddit user, it's mostly because there are no other alternatives. I definitely would not buy their stock.
1. Relatively easy to create a subforum. 2. Someplace you can post [pseudo]anonymously. 3. Has a critical mass of people. 4. Is accessible to anyone on the web.
There's a subreddit for all sorts of topics, and if there isn't one, it's fairly easy to just start one. You don't have to deal with setting up forum software or paying someone for it, and if you want to you can create a post with a pseudoanonymous throwaway account. It's really easy to come and go, and there's enough moderation that it hasn't in general devolved into a *chan-style environment, but not so much it's alienating.
Pseudoanonymity is maybe underappreciated in social networking I think. It's really a key feature. Once you reach some mainstream critical mass, having that feature really moves things along.
If you could replicate those four things in a decentralized way, it might take off. But there's a chicken-and-egg issue with network effects.
and that most active & upvoted redditors on a sub mostly don't use its discord
I added 'single login' as an after thought, because it didn't occur to me at first, but I think that must be a big reason. But even then there's Discourse, with not single but much easier and OAuth (signup/)login.
So, yeah, reddit is the searchability.
A "classic forum" trying to get off the ground would be competing with all of that. You could also argue, "why Discord? Why not IRC?" for much the same reasons.
As I said there, there is also Discourse, with only slightly more multi-community friction really, making it far easier and more standardised to do initial sign-up.