Alrighty.
So, let's kick off with the American first part. There is a wide range of RP out there, and not all of it in English. Just in the fandoms I actually know anything about, I know sites in English (obvs) but also in French, Brazilian Portuguese, Spanish, Polish, Hungarian and Czech. Jcink's support for non-English languages... it doesn't exist so before you begin you not only need to skin the site, you also need to translate all the parts of the site you can. Or you could go to, say, Forumotion which has this built in, especially under its Forumactif brand (which is essentially the same thing in another language)
And that's before we get into the culture wars. I'm still amazed at how the purity wars are such a thing, especially with being told to my face that 'white is a single ethnicity' by one of the admins of CTTW... like, come to Europe, I will soon show you that white isn't a single ethnicity - and no-one in Europe would take you seriously with that claim, not even close. That angle, plus the sheer amount of super-vocal posturing that goes on... the term BIPOC doesn't really exist outside the US because the I part isn't exactly being targeted or marginalised, because that's just not an angle of factionalism that we have in Europe, nor is it present in the same way in large parts of South America. I could go on but I'm sure you get my point here.
As to the white - see the culture wars. See all the stories of white people virtue signalling their inclusivity and steamrollering over the concerns of actually marginalised groups who, you'd think, would know their own concerns and struggles.
Predominantly female? Of course it is. Me, I simply don't admit to my gender any more because as a cis-het white guy I'm straight up not welcome. See the debate on jcinkanon (January thread, page 3 I think?) where it's discussed how unwelcome guys in general are.
The 20s demographic, that's easy enough to infer mostly from the factionalism going on with the age debates, e.g. pointing out that using xD is cringe (sorry it's not xD these days it's 😭, or perhaps actually 💀 and I just dated myself by using the term cringe), but also the subtle shift in discussions towards a 25+ minimum age in RP because it's not already gate-kept enough.
But the single biggest thing I know is how small Jcink really is in terms of the RP scene. The animanga folks don't go there (they're not welcome), the predominantly male folks with wolf ambitions don't go there (they're not welcome), the folks who are quasi-refugees from TTRPGs don't go there (they're not welcome, also predominantly guys), the folks who are more technical and want to experiment with doing more customisation than you might think at first glance don't go there (they're self hosting for the most part, also predominantly guys), and that's before you talk about languages and features.
That's the thing, I've helped out individual self hosting RP forums that used to get more posts in a single day than many RP sites get in years. I used to help out warriorcatsrpg.com before it rebranded to Feral Front, back when it was still only a baby site at 25 million posts, I helped the server admin tune it, at its peak it was getting more than a post a second on average, at 90k posts per day (vs 86400 seconds in a day). I've lost count of the just-SMF self hosted RP forums I've helped over the years, and they're not only invisible to the Jcink RPC, they actively stay away because they know full well they're not welcome to share the space, so they don't even try.
I also think it's funny; my wife started a HP RP back in the day (like, 2000 on a hosted service, went self hosted in 2003) and gave up admin to someone else in 2014. That site is still running, it even moved to Jcink for a brief spell (less than 6 months) but they actually moved back again because they didn't have the features they liked from their self hosting. And they're still trucking along, over 1000 posts since they moved back in October 2022. They're not ever going to be as busy as they once were, and the 127,000 posts they currently have is a fraction of what they've had in their lifetime, but that's the kind of thing I mean. I also wouldn't be surprised if they got fed up of the moralising that the Jcink ecosystem seems to do - because other wider-community spaces *don't*.
Fun fact: when I first started working on StoryBB, there were several other people in the project. I was told, repeatedly, that the use-case for what is essentially the Jcink RP use-case was 'so small it doesn't exist'. That I should be building much more like RP Nation where the one forum hosts many games across many different things, rather than the one-site-one-RP model. But these were people who wouldn't have been welcome in the Jcink arena even if they had wanted to become part of it - which they never would have.
And it cracked me up that the founder and lead developer of IPS had never heard of Jcink and was genuinely surprised it was a viable business model the way Jcink is set up. But given that as far as I can tell there's only maybe 2-3 physical servers based on the IP addresses and the visible A records, it probably doesn't cost that much to run.
I will also note for the record how bitterly StoryBB was received. I realised it wouldn't have mattered if I'd built out a hosting service for it, the bulk of the Jcink RPC doesn't care, doesn't want it, actively shuns alternatives, sneers down its nose at self-hosting etc. but fortunately I realised that the Jcink RPC was not the whole RPC, hence the resurgence of energy in the new year.