Imho it is not reddit forums are competing, but rather discord.
Another imho - for more active forums, there should be a better multiplayer support.
Single player people come to forums, mostly when they encounter bugs. They rarely discuss stuff happening in the game (especially if there is no campaign).
Multiplayer people are however a stronger backbone, as those people return to games more frequent, and therefore they care more. And since they care more, they spend their time on thinking about stuff around the game. And once the process of thinking has been done, a post is being created on a forum.
Forums are a place of interactions. Multiplayer people are the ones, who are willing to interact with other people the most.
Therefore - the bigger the multiplayer - the more people willing to interact with other people.
So in the end, it is not as much about what the forum misses, it's more about, how to get more people into multiplayer, so that there are more people willing to interact with others.
I love forums and hate multi-player (specifically in the case of 4x games). I don't think people not liking multi-player keeps them from posting on forums at all. There are many, many forums that are mostly, if not entirely, filled by single player game people.
I do agree that a big chunk of the competition is discord. But the thing is, discord is terrible for long term solutions. It is great for communication between devs and the community, especially in early release situations where both quick and back-and-forth conversations are ideal. I just don't see people going to discord and doing searches to find answers that have long been answered. EVERY person asks the same question over again. Reading the FAQs that may be on the Discord servers, lol, unheard of!
Reddit is kind of the competition of forums, but one of the biggest actual competitors are the Steam forums. Many gamers expect and demand (sometimes in a very condescending and/or entitled way) that every game has to have a Steam forum and that forum MUST be the main forum for the game. If that is the game devs decision I am perfectly fine with that. But I am also fine with those forums being just what they are labeled by Steam.. which is "Steam Community Forums" with emphasis on the word Community in my opinion. And a game having their own website forum setup is great IMO. Another password... meh, big deal, that is exactly why I use a password manager.