In many cases it's not so much skill (once you're at a reasonable level) as it is time, which makes it difficult to please everyone.

Okay, you need to be able to understand and follow tactics, and know how to play your class. If you stand in pools of poisonous green goo and ignore the adds, you're not going to get very far. Equally, if your DPS rotation consists of spamming the first skill you learn, you tank exactly one enemy at a time and you heal as a BLM casting Physick then you're going to have a bad time. Barring that, once someone somewhere has worked out a decent strategy and your gear is acceptable, your success depends largely upon whether you can execute that strategy without making too many mistakes. That might require a fair bit of practice, of course, but you should get there eventually.

There's always going to be a limited amount of content, and the rate at which it's created is never going to come close to the rate at which it's consumed. And so gear starts becoming a limiting factor. Sometimes a hard one, because you'll inevitably fail a DPS check, sometimes a soft one, because as your gear improves, so does your effectiveness, so battles become more forgiving of the occasional mistake.

Practice is time and getting gear (through repeated dungeon runs) is time. "Hardcore" generally means "I am willing to dedicate a lot of time to this game in order to get to the top". If there's an ultra mega weapon that you can only get if you run through some particular dungeon 200 times, there are people who will run that particular dungeon 200 times as fast as they possibly can. They might complain about it a bit, but they will knuckle down and do it, and some will get there within a few days. If the next dungeon pretty much requires you to have that ultra mega weapon, you're gating that content to only allow people through who are going to spend many hours running through the previous one. And the next dungeon probably does need to require the ultra mega weapon, or there's not going to be much point in getting it, given that it doesn't set your achievement in getting it apart from people who ran that dungeon once and then carried on to the next one. Making that later content feel exclusive gives you a reward for all that effort. And the designers really need to try to hold these people back a bit by gating content to stop them burning to the end too quickly, at which point there's nothing for them to do, and they might get bored and unsubscribe.

But then there are people who might like the game a lot and are pretty competent at playing it, but only have time for a couple of dungeon runs a day. Maybe they have other commitments, maybe it's just one of several things they're enjoying right now etc. But they enjoy playing new content and learning new strategies just as much as anyone else. It just takes them longer, as they have fewer hours a day to dedicate to doing so. They get up to the point at which they pretty much have to run that same dungeon 200 times to continue, and, ouch. Sure, they'd like to see what's on the other side. But facing weeks upon weeks of repeating that same dungeon without moving on? They're probably not in a raiding FC that's going to guarantee a good group in return for 3-4+ evenings a week's commitment, either, so even if they have the tactics down, they might be regularly DF'ing with those who are still learning, further slowing down their progress. They could just try to bypass it and move on, but the later dungeon is just too hard without better gear. What can they do? Faced with never really progressing for the foreseeable future, they may give up and unsubscribe.

SE's answer seems to be to relax the requirements as time goes on, which makes sense. But it's certainly a difficult balance to reach: stop people who have large amounts of time running out of content too quickly, while stopping people who don't have as much time from hitting some near-insurmountable wall. I don't think anyone sensible wants high-level gear to be handed out for nothing, because then everyone is sitting at the top waiting for something new, and there's no sense of accomplishment in obtaining something that you haven't worked for. But the amount of time that different people are willing to put towards getting there is very variable, and everyone wants (and, in many ways, deserves) to feel some progression in the amount of time they have.