It's in either one of BC interviews or the Q&A right after it.
Ion actually didn't like that people called them subraces and made a remark that they weren't. Allied races are just, well, some allied races, they only need to have some backstory that allies them w/ either Ally or Horde. They don't need to be closely related to already existing races in terms of ethnicity, etc.
It's totally expected that some proper races won't get any "subraces" at all whereas others will get more than one. Which I kinda don't like because something like HM Tauren is made into a separate race rather than a set of extended customisation options for normal Tauren, the only difference is tats and antlers. Same goes for DI Dwarves and LF spacegoats actually.
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They could give red spacegoats to Horde
Also, by your logic, you wouldn't need LF spacegoats. "You already have LF Draenei, they aren't infused w/ light and are called the Draenei..."