Certain types of mechanics tend to be particularly difficult for lower-skilled groups, and they're not usually the mechanics you're looking at.
Target-switching is one of the biggest issues. I'm really not sure why - this one is purely observational. Beginning raiders are very bad at target-switching.
Interrupts are another issue. They're highly timing-sensitive and they require monitoring an additional UI element. Interrupts that involve target-switching are probably the single most difficult single mechanic in the game for them.
They also tend to struggle with keeping up DPS while moving to a much greater extent than more skilled players do. For them, it's not just a matter of the game's restrictions on what you can do while moving; it's a matter of them having trouble doing anything at all while moving because of physical coordination/mental multitasking shortfalls. Navigating the game world - especially if they have to strafe - is itself a relatively complex task.
Multitasking in general is a big issue. Whereas more skilled players have, to one degree or another, automated and 'chunked' common basic tasks like doing their rotation and watching for/moving out of fire, less-skilled players still consciously manage those basic functions. That conscious management leaves less working memory available for higher-order tasks.
And they tend to have trouble with complex priority systems. Simple stimulus-response type mechanics don't tend to be a big issue, but complex mechanics like long target priority chains, conditional responses, etc. are messy and often produce much slower reaction times and lower accuracy.
When a higher-skilled player tries to evaluate encounter difficulty, you'll usually look at things like healing requirements. You might also consider raid coordination requirements (e.g. Mahourai's comment on Empress - it's true, Empress does require a relatively high level of coordination for a normal mode, and that might increase the learning curve by a pull or two). You'll consider the sorts of things that make encounters difficult for good players.
But base healing requirements and coordination don't actually matter that much to less-skilled players. They add about the same amount of difficulty regardless of skill level. (Actually, perversely, phases with high healing requirements often reduce overall encounter difficulty at lower skill levels because they're not usually coupled with other high-difficulty mechanics. Rampages are the easiest part of the Megaera encounter, and Primordius is easier with the zerg strat.)
So basically, you're looking at the wrong things. You're not seeing what makes these encounters so difficult for newer or weaker raiders, and so you're not seeing just how tremendous the jump in difficulty is from t14 to t15 for players who struggled with t14.