Rexxar is half-Orc/half-Ogre as I recall, which is why he looks so darn weird.
It's because they went with pretty much the most iconic variant to each class and slightly by story relevance to my knowledge.
So there are 3 Orcs (Gul'dan, Thrall, and Garrosh), a half-Orc (Rexxar), 3 Humans (Anduin, Uther, and Jaina), and 2 elves (Malfurion and Valeera; the former Night and the latter Blood).