Hi, someone who considers himself fairly casual (though, to be fair, "casual" is a very nebulous term and others may consider me something more like "lazy midcore" or whatnot) here.
I don't know how serious you were or weren't about this request, or if you were just venting, but I'd argue that it would be hard for such a "player base" that does not necessarily self-select into that category would have any sort of representative that could accurately represent even a small fraction of the players that might fall under that umbrella. That being said, I'm happy to offer my own thoughts. Of course, it's only personal anecdotes -- not very helpful in the grand scheme of things!
To start with, my sample size is quite low -- only three or four runs. 24-man content stresses me (and my laptop, haha) out a bit, so I don't run it much unless I'm specifically invited to do so (and even then I often pass). However, I've never died on Hashmal personally. And, to be frank, I would attribute that mainly to playing Bard and being able to pump out my rotation both on the move and at a distance. It seems much easier to keep track of the battle from the sidelines when it comes to Hashmal, as opposed to standing around in the middle as a melee and having to sweep around on all sides to see what's going on.
Another strong factor in my favor is that when I go, I'm more often than not on voicechat with more experienced players who are happy to call things out if need be, although mostly it's just general chatter going on. Still, it really helps me stay focused and for my first run especially it really smoothed things out. Many players probably don't have this advantage, for one reason or another. Tied in with this is that I'm running with a party (or at least a few members) I know I can rely upon.
However, even though these are players who have cleared the current Savage tier, some of them still end up having a nice chat with the floor during Hashmal. Which I guess brings me to my point: I don't think this is a problem that is expressly related to "muh casuals!" I'm perfectly willing to concede that there are plenty of more casual players who simply aren't as skilled as more hardcore players, and that of course will lead to slip-ups and underperformance. I don't think anyone would tell you otherwise. Additionally, more casual players might tend towards having slower reaction times or having to think more actively to recognize mechanics than being able to instinctively react to them, which can also lead to mistakes. I guess the idea is "everyone makes mistakes, but people who don't practice as much naturally make more." But a problem that both groups can share in these situations is, well, lack of effort.
Or maybe, more accurately, diffusion of responsibility. The more people there are, the less personally responsible each individual will feel for the group's overall success. I think that's pretty natural. However, on all ends of the player spectrum, it can lead to complacency and laziness. "Oh, it's okay if I flub this mechanic, the healer will get me." "Well, it's fine if our alliance wipes, the other two will pick us up and/or finish it off themselves." Those kinds of things. Maybe more casual players feel like they can get carried. Maybe more experienced players think it's a good chance to goof off a bit since there are more bodies to pick up the slack (and, let's face it, Rabanastre isn't that difficult, so I doubt they'd feel terribly obligated to face up to some sort of challenge). Plus it's much easier to hide one's mistakes amidst the chaos of the fight and 24 bodies, so you're less likely to get called out on that sorta stuff. In fact, I suspect part of the reason some players might struggle to learn is that it's harder (at least to me -- maybe not for more experienced/observant players?) to identify who's struggling and for what reasons, and thus they don't get as much personally directed advice. While anonymity protects you from abuse (to an extent), it also prevents you from seeking ways to improve (to that same extent).
I have no idea if this answers any questions or provides any insights, but I thought it couldn't hurt to put down some thoughts, disorganized as they might be (I've got a bit of a headache going, sorry!). So...yeah there's my little spiel, haha.
TL;DR: I don't think it's some mystery specifically related to Hashmal. It's the same ol' problems you'd see in anything else. Hashmal just isn't as horrifyingly outgeared as the other stuff yet so the mistakes matter more.