Death knights are not so broad a thing that any old necromancer will fit them. Player DKs come form a very specific place. And while yes, blizzard can do whatever they want, the amount of pandaren running around the world pre MoP were INCREDIBLY FEW. Like, it makes the remaining high elves look heavily populated. While it's POSSIBLE one or two could end up in the hands of the scourge, the idea of there being a sizable number of pandaren caught up in it at the very specific point in time the death knight starting experience takes place isn't likely. Possible but it would be a bigger stretch than Tauren learning a new fighting style.