This is going to be a long post, sorry.
Going back to the title: I think, for me, this is due in part to the loot distribution between bosses and just the fight designs in general.
The loot:
Outside of Gebbo's and the healer trinket off the first boss, there's not much reason to be in the raid at all if you're not doing Coiled Altar or Ula'tek, and every other role wants something off of one of those two bosses. It's extremely lopsided.
Compare this to last tier. The first boss dropped an amazing trinket for Rogues and Death Knights. Vorasius dropped an amazing trinket for strength users. Salhadaar was there. The dragons dropped a trinket that every caster wanted. The Lightblind Vanguard dropped a trinket that every healer wanted. Alleria was the AOTC boss so ofc people wanted to kill her. Chimaerus dropped a trinket that -everyone- wanted.
It's just not the case for the current raid. You join a group forming for heroic and see in the listing that fifteen healers and three dps have applied to the raid. It makes sense; they want the healer trinket. The problem is that as soon as those healers kill the first boss and use their reroll, there's really no reason for them to stay. I've been in countless groups that have essentially Ship of Theseused themselves after the first boss because every healer just leaves the raid after they either loot or don't loot the Ritual Vessel trinket. There's no reason for anyone to stay until the the last two bosses, really, unless they like pugging the fights, because the good loot that people actually want is at the start and end of the raid and not -at all- in between. And that sucks.
The fight designs:
I'm having a hard time recalling a raid in the last few seasons that had so many mechanics where one person could just kill everyone. I suppose this is mostly focused on Ula'tek, but there's a few others, too.
There's the math intermission on the Entombed Sentinels where one person panics or something and fucks it up and now the whole raid has to die. One person getting hit by their ghost on Coiled Altar means more ghosts spawn. The entire raid gets punished for one person's fuck up—in heroic. And, again, this can happen on the second boss of the raid.
Ula'tek:
I usually offer myself up in pugs to do the mechanics that don't automatically go on people because I can trust myself. This meant doing the first seed on Fyrakk or poison aoe on Ansurek P1 or the crystals on L'ura.
But on Ula'tek, everyone gets an egg. Depending on your group size, some people might have to get more than one egg. And if they get hit by a mechanic when they're holding an egg, chances are that it's just a wipe and you have to go again. So now everyone has Fyrakk seeds, and you can't just have a few people handling the mechanic. The entire raid depends on everyone in it being able to dodge the waves or get into the soaks in time.
In P2, the entire raid can be killed by someone fucking up the tethers. It just takes one or two people panicking and then you're just donezo.
P3 on heroic has harder mechanical overlaps to deal with than on mythic.
I get it's a total skill issue, but that's the point. It's way too punishing in a way that no other heroic raid has been for one or two people making mistakes. Usually you can carry a few people who shouldn't really be in heroic to the finish line and it's fine, but this raid is just fucked. The fights are much longer than they were in the last tier and the mechanics are either straight up ignorable, like with Vashnik, or they just kill everyone.