I think that's because in WoW, fashion and transmogrification didn't come until muuuuch later on. You'd literally go for stats rather than looks. As for the orcs...back in the day they weren't smart enough to play any of the casting classes so they shouldn't have been wearing any robes. :P
WoW did have massive differences in how each gender wore certain bits of gear; suit of armour turning into an itty-bitty-plate-tankini for example. Even in WoD they still do the gender differences in robe-like gear (but a lot less extreme):
Personally, I have no opposition to this. Instead of have two separate chest pieces that are gender-locked, you would simply have one item, that changes based on who is wearing it. Takes up less space and gets rid of gender-locking (as you would no longer need separate items). It would however mean that a female would never get the males look or otherwise, unless they changed gender. I'm fine with this, however I can imagine many people wouldn't be.
Neither option is discriminatory though.