Yeah, but they're not playable Orcs. Your Orc hero actually begins his adventure way before the Iron Horde invades. Iron Horde Orcs have a completely different history and background than your standard timeline Orcs.
We'll simply have to agree to disagree here. I feel that limiting it to Goblins and Gnomes makes the concept far stronger. When you start putting Tauren, Blood Elves, Humans, Orcs, Worgen, and other races in mechs, it starts getting VERY stupid. It looked cool when Gelbin was piloting a mech because we're used to that character being surrounded by technology. Like so:Let's compare:
Many races have their own approaches to tech, but can be condensed into a solid class. Why not? Now take priests, for example. Each race has its unique approach to the light, yet they're still all priests and share the same classes. By that logic, Night Elven priests should be Moon Priestesses (they are, in lore, but not really in actual gameplay), or Undead only Shadow Priests, or... do you see the logic?
The more races involved the richer and more flexible the class is in terms of gameplay, which is what matters in the end. Barring up a wide and rich concept to only Goblins and Gnomes renders the class poorer.
Varian or Genn piloting a mech? Not so much.