Ancient topic beaten to death.
Truth is simple. In the earlier 00's, social media wasn't such an overwhelming force. Centralization of the internet wasn't either. Nor were smart phone apps, containerized internet experiences and the like. All that's happened now and trends for what people tend to do on the internet have changed.
Traditional forums, BBS and the like, have seen a massive spiral. Only the biggest ones, usually the type peddling products of piracy, still have real activity and have gobble monstered many thousands of their lesser variants that saw life "in the day." Then of that, a fraction still are RP oriented. In that, you typically see the ones with any "active" status be things like erotica.
Notice how many people coming here mention "I came from Discord RP" or "I came from Reddit."
It's just how things have trended. Doesn't help what lingers on in the pbp RP space is battered with nonstop pop-politics wagon dragging and nonsensical clique based drama, there isn't exactly a great "mentorship program" to onboard and keep the concept thriving for new generations.
Even with all this, it's still around and alive. I don't anticipate forum based roleplay will truly disappear anytime soon. Once people stop bellyaching about LLMs, I imagine that'll end up driving some minor growth into the space, if not here then elsewhere. Otherwise it'll retain its niche status until something technological makes barrier of entry such less significant in effort that more people poke at it.
Myself, I write when I want to write and don't when I don't. I only first came here to make fun of LeeRoy for losing in the Arena. Shock and amazement my extremely slow-burn RP on here is currently in the Arena, dealing with not one or two but nine disgruntled adults RP fighting. I see RPG as a fusion of roleplay on the forums and a conduit for collaboration on Discord but, doubt the sweaty arena stuff'll ever not be my focus.
TLDR: There's no perfect solution to any of this, just write.