Because comparing a 710 DH tank vs a 745+ Mythic HFC geared tank in tanking a Mythic HFC run is balls out stupid. So compare apples to apples. 710 DH in a Heroic with a similarly geared Other Class Tank = No issues with aggro. 710 DH tank in a Mythic vs a fully geared tank with a 795 ring and all the trimmings = Yeah, issues. But that's not down to the class.
K, I need to stop replying to this thread. It's full of so much derp I'm going to blow some braincells.