The same reason like in retail, when raidgroups cannot clear the NHC-mode. Bad people, less time, ultra-casuals, and so on.
I played WoW since the beginning, raided, pvp'ed, was in Top100 world raids (For top20 too less time, although I was a student), a few times gladiator later in arena, so I think that the Vanilla bosses are the easiest from my point of view. There definitely people saying the opposite, but all of that, including my comment, is a personal opinion --> Vanilla was the coolest, simply because people used to know each other and play with or against them. That was the funniest part in Vanilla, till the whole server transfer, name/race chance, faction change paid idiotic services were announced!
Neither the raids are hard (ok, back then they were really hard, but we were all worse players then now, clicking the main spells, no other keybindings, less addons, no knowledge or youtube guides for every boss..) nor the pvp is hard, but time consuming.