Because threat no longer exists

Because threat no longer exists for non-tank specs in raids. Tanks are the only spec that can taunt, and it is impossible to pull agro from them once they start the encounter.
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Apparently the developers thought threat was too difficult, and have been progressingly making it irrelevant since Cata, and this will be the ultimate outcome -- threat is simply something healers and DPS no longer need to worry about, ever, in any raid encounter.