No one wants to share or play as a team, knowing that loot is whole team effort not just yours even if you are top dps. If you think otherwise try killing any raid boss of Wrath alone… Well maybe a BDK can kill some bosses of Naxx allready, but you get the point.
Loot system was poorly rewarding in Vanilla raids, when you had 3 items per 40ppl in raid drop and loot was class specific, so GL if Alliance didn’t had shamans with shaman tier pieces and Horde with paladin ones. Oh and considering all raid full of rogues and warriors good luck with their tier sets too.
Ever since t4 we had 4 pieces per boss for 25 ppl, but Ulduar HM loot system broke that. After couple of months everyone got every 232 piece they wanted and hunt for 252 items begun, reducing usefull loot pool to only 1 item for 25ppl from a loot table that can have 5-6 items. Phase 3 should had have if not difficulty nerf then at least making so loot in HM kill was HM quality.
On top of that Blizzard also killed 25ppl normals with changes to the Ulduar loot in phase 3. There are now way too many easier and faster sources of 232 gear than in 25ppl Ulduar and 245 gear in TOGC in a faster and easier raid.
I proposed a solution in another topic