It's because Mythic is all a numbers game for the most part.
Your tanks are either getting clobbered (which most of the time they are regardless of everything else)
Or you're hitting enrages. Of course some bosses are push overs but when it matters, it usually comes down to surviving by the skin of your teeth. I reckon their DPS is amazing so they want their tanks to be more self sustainable.
I'd be surprised if they didn't switch to Chi Ex for farm, save for the few bosses where Serenity is just the better choice.