Mm. Think the idea was to alleviate the shortage of tanks by having an entire class that would be able to tank no matter what spec the player chooses.
Obviously 98% of the DKs refused to tank despite that, and for the remaining 2% their groups wished they just refused to tank.
So now they've just chosen to make a dedicated tank spec. At least now, when you get a DK tank you know he made a choice to tank (and collect gear for it) So hopefully the average DK-tank won't be as wtfbbqbloodybad.