#1 Dungeon scaling is a huge part of it. If there is ONE part of a dungeon that I am worried about in a quest, it is tanking a trap on a non-evasion character. A trap hitting my 200hp character for 150 is fine…250, not so much. Elite has never scaled as much is it does now. It is really insane. Elite should NOT SCALE, EVER, AT ALL. Worrying about mobs/casters/traps hitting NOTICABLY harder when another party member enters a quest is just silly, particularly on elite. When the smart thing to do is ask the party to wait outside till you hop past a trap, something is incredibly wrong.
#2 Bravery is part of it. Instead of Nx8/H/E, the elite run is first. No time to build a rythym, no time to learn for newer player. Know it or die. Self heal or die. Be skilled or die. All that leads to not taking noobs/unskilled/ungeared players and non-selfhealing classes. Coupled with point #1 this results in smaller partys with everyone being self sufficient working better than a traditional 6 person party that doesn't know the quest.
#3 Attitude Many top players think…if I can solo elite most of the time I am not pugging anymore. I used to, but it is just slower, or -10%, and for what? Most puggers I carry through a quest and then they complain…about me not waiting for their late penalty, about me zerging ahead, about me not showing them the way to the quest, about me not sharing a walkup right away, about dungeon alert…about the lack of a healer when the lfm said bring your own, complain about not letting them running through the entire quest to loot a random lootgen chest when the party is waiting holding the window, etc. Yes, I know it is a game…but if you put in lfm that it is an xp focused party then please respect that…it's only fair, I don't join flowersniffing/no zerging/wait for full party/roleplaying etc groups and inflict my playstyle on them.
I usually still pug anyway, but most top players (TR or endgame) have gone guild/channel only. People respond to incentives and for the elite player pounding out past lives that first life barb that asks "what house" "share" when joining a vale quest is at best useless and at worst trouble. The incentives for pugging have been reduced to knowing you are doing something good for the server…and potentially meeting some cool peeps…in terms of the numbers…not so much, and a lot of people care about that quite a bit.