LFR probably is extremely boring for real raiders. And on top of it half of the raid is probably afk looking facebook or w/e while the tank is tanking. The tank is the only one having to stay 100% focused.
I think this is the major reason that tanks don't wanna do LFR anymore, because players can afk and get away with it while he has no choice but keep tanking. such a let down.
Another problem is that LFR content force real raiders to carry it, because it's completely inappropriate for non-raiders. In the end, forced raiders (tanks) + casual players is not a good equation. Pretty sure tanks don't build tank only to tank LFR content. Now, they have an alternative, and many will *never* do LFR again.