same reason a short legged with a beer gut panda bear race has a tanking spec which is based on drinks , kegs and some hands and foot work, as if a panda bear tanking ragnaros can really avoid all the fire dmg and ragnaros can really miss hit him and all that while being drunk.
if up untill now our tanking classes had some reason behind the concept, all but 1 used plate , 1 leather could trasform into a giant bear, some needed shields , now days you dont need anything to tank, have a drink , do some empty hand fancy movments and some fancy foot work and your a tank!
just check the cinematic of MOP , the first ever class that can put to shame not one, but two other classes/races, a new way for blizz to show us whos the new GOD mode is...
so why not let locks tank and DKs to raid heal....