The reality is the explanation is Doylist. They wanted BElf models for Alliance, and any time there's a new race/class/model, they feature them for a bit as a form of advertising. If the NewThing is enough of a hit, we see it featured more. If not, it goes the way of the pandas.
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Do you want the Hordies to stick to lore (good luck), or are you prepared for twisted things like "The Draenei are responsible for the Legion burning every world they fled to!"?