Same thing happened with ICC when Cataclysm came out. The optimal players did the content while it was current and had little incentive to do it again, especially when they could be working on the new content. Even if they could be brought back to do this content, they're not likely willing to wipe repeatedly on what is trivial content due to gear and level increases just because the people most likely causing the wipes weren't willing to put in some effort into seeing how the fight was done while current. Using DS for mechanic failure examples:
Morchok - Stomp + Crystals
Slime boss (Yorsahj?) - Not knowing what abilities the boss gets from which slimes reach him
Orb boss (Zonozz?) - Not knowing how to bounce the ball as a raid, not knowing to kill or quickly kill adds during black phase
Hagara - Frostflake, Ice Tombs, Lightning Phase
Ultraxion - Fading Light has a varied duration (3-5 seconds I think on heroic), Heroic Will has a 5 second cooldown, not realizing that eating an Hour of Twilight will kill you if you have the debuff
Warmaster - Letting the ship die by not stacking for the big blast, or soaking little blasts. Not knowing that eating a small blast increases shadow damage taken for next X seconds, not killing both drakes before they take off, Shockwave, Warmaster's Roar, not killing the dragon first
Spine - Rolling, killing too many tentacles, not stacking bloods onto one amalgamation, not popping cds to kill tendon
Madness - Platform order, blistering tentacles, Bolt + Explosion, bloods