It's easier if a healer screws up and dies than a tank. The tanks face is now 15% weaker while getting pounded as hard as before, causing both healers to work 15% harder as well and probably getting you killed in progression raids. Ifrit EM RNG's you with res weakness? Have fun trying with 30% less stats.

If a healer dies, the other one simply resses him and the bard sings a song. Healingwise you are now only down by 7.5% of total.

And due to every fight needed tank swaps lately, during your 'downtime' you are basically having to perform the dps way of fighting the boss (over say titan hm just standing there). On EM a tank has to do the dps job half the time and tank job the other half, thats more responsibility by definition. Healers also need to see tank bursts and swaps coming but as said earlier there are 2 of them to react to it, while the tank has 100% of the responsibility here again. Other tank cant pop cooldowns for you.