Forums* in general are just not very popular anymore in terms of discussion and socializing. There's better and more preferred methods for general and gaming discussion e.g. Discord.
Sythe's market is still active because Discord has strict rules against cheating communities, so users find value in that feature despite the archaic format that is web forums. Sythe was born on the premise of cheating, so it continues to be a haven for those interested where more popular formats are to risk-adverse to allow cheating.
The general discussion community will never be what it was in this forum-based format because of better platforms like Discord though. Older generations who had an emotional attachment to Sythe are long gone, in their late 20s, early 30s, having families, working jobs, and having no time for the edgelord toxicity that stayed behind here. Newer generations prefer a consistent location for their various communities. I dip in now and again out of curiosity and am always reminded the golden age is long gone.
There's also one specific paradigm shift that Discord and Reddit offers that Sythe could never: self-administration.
When a new trending game comes out, Discords channels and sub reddits can be spun up by just about anyone. Many channels will be created, a few will boom, one or two might solidify into an active community. All you as a user have to do is find them and join the channel, and then you can just context switch between channels depending on your interests on an given day. Sythe, conversely, is infinitely more rigid. It's the same stale view day after day. You don't just get to spin up a new subforum. You don't get to take a stab at leading the platform into a new trending game to capture that initial buzz. All you get to do is submit a suggestion, wait for 4 months for it to get approved (or likely denied), and IF it's approved, then an administrator needs to create the forums.
By the time ALL THAT is finally accomplished, Sythe will have missed the buzz. Interested gamers have already found their preferred channels elsewhere, there's zero incentive to switch to sythe for an optically dead subforum. For example, Pal World. It went from 2+ million concurrent players for a month straight, and barely 3 months later it plummeted to an average 100K or less. Sythe could have NEVER captured that new community because the buzz would have died by the time they spun up the forums to capture the new users.
And that's why you rarely see new subforums created here: because it makes no sense to try and beat the rush when Discord channels can be sprung up en masse with little to no effort to soak up that buzzing interest. Sythe's only possible play is to wait for a game to prove itself to be a gaming staple, and THEN accommodate their EXISTING users with some forums on the subject. That's not a growth tactic, that's an appeasement tactic for the current (shrinking) userbase.
tl;dr sythe has no means to attract new social users outside the market; any new users are often drive-bys with no interest in a community outside of their discord world, and just want to purchase some cheats.