Would it be nice, in some cases, if the raiding population was higher? Sure. But you can find a static, a pug, for just about any turn on any DC including Ultimates. NA also has a tendency to play a lot riskier than JP, and I have a feeling the lack of utilizing macros for quick pre-pull positions and strategies intermixed with people who'll just drop from a party after a single screwup in a learning party contribute to the overall issue on some level. Becomes kind of hard to learn a fight when people rage quit after a shiva wipe in E12s when the PF listed is specifically for practicing and progging that specific mechanic. All that, and, PF can take a while to fill whereas queuing into a duty multiple people have down to reclear is, probably, faster. Are those the only reasons why JP would have a higher overall clear rate/raid participation? No, but some of it probably contributes.
But it doesn't really impact me if Chocobo has a ton of clears, Crystal tends to have the least due to new players being often pointed to it intermixed with it being predominantly a data center for RP and people pointing away from it as a place to go if you want to raid. And if people more interested in raiding content join a DC known for having a more active raiding population, the other DC's raiding populations tend to stay lower as more raid-interested players will opt for the DC with the higher population for the content they want to do. During the DC split a lot of the raiding population left to stay on Aether, usually since their statics were staying there (which was my case, initially), but I'd say the raiding population has kind of been rebuilding slowly. It's at least more active than I remember post-split on an alt looking through it.
PF is just PF at the end of the day on any DC. Aether PF can be just as messy as Crystal PF, Crystal PF can be as clean as Aether PF. One just has a smaller overall population, and tends to utilize different strategies (like how Aether seems to lean heavy towards Line Lions, while Crystal tends to lean heavily towards Billi Bili) and a big thing with the smaller population is that it generally takes a lot longer to fill a party for a fight. For statics, a lot of people really interested in high-end raiding (ie: speedkills, high logs, hardcore statics, world first groups and so on) will generally transfer where there's opportunities to do that. And in the reverse, people more interested in RP content will usually xfer eventually to Crystal if they don't have a particularly high motivation to raid.
And, the obvious thing, if the raiding population is actually smaller (or a lot newer), that'll also impact clear rates pretty heavily. But I figure that should go without saying.
I'd say a primary way to increase raid participation would be a community oriented thing to work on, providing easy to access and clear resources (which do exist, from SaltedXIV, Ahkmorning, The Balance, various content creator Discords like Momo Sama's, Rin Karigani's etc... and creating a more open space for people to ask questions, or have an easy "in" to start out with -- like Rin and Llama Todd, from what I know, tend to host a number of learning parties here and there to help people get into raiding, and pretty much any high end raider who makes video content, or guide content in general, tend to be more eager to answer questions and so on. Back in StB I know a lot of streamers -- at least -- tended to join clear parties to help out and provide information if asked/be that little extra edge to help push an otherwise capable group over the line which, sort of by proxy, provided the same advice to anyone watching them who might have an interest in the content and are checking it out that via their stream first).
But it's not a catch all, really. At least, that's my actual opinion if the thread's supposed to actually generate thoughtful conversation about the subject.
I mean, when I xferred back to Crystal I somehow shitposted my way into a TEA fresh prog pug for fun and, within half a lockout, we saw LL enrage. That's anecdotal and personal experience, but it does kinda stick with me a bit. Made me want to go back in, or at least do Ultimates in general, but I just tend to be a bit on the shier side so I haven't really committed to looking for a group.
Though, just posting graphs repeatedly and adding nothing isn't a conversation, or insightful. Kinda why I figure this is all just a troll, but I don't know, it's a topic I do somewhat think about from time to time and is kind of worth discussing. At least, more worth discussing than idk, Viera hats again.