People always say it is because of Reddit or Discord, but that is a description of the problem, not the underlying reason. What is it that makes Reddit or Discord superior to forums?
Personally, I've always preferred forums. They allow for more permanent discussions, people generally put more effort into their posts (not everyone all the time, but on average) and they are a lot more structured. You have a nice neat division into different subforums in which discussions are again structured by threads.
In contrast, Discord is basically just an assortment of chats that flow aimlessly and go nowhere. Finding stuff is often difficult, too. Most posts are just effortless one-liners, spam, emojis or trolling. And Reddit isn't perfect, either. The upvoting system usually promotes in-jokes, memes and even trolling. Having a proper discussion is often basically impossible. Voicing a dissenting opinion in a subreddit often leads to being downvoted to hell. Not to mention the arcane posting restrictions in most subreddits.