Because nobody would actually play one.
At least, not more than an hour.
Ever get killed playing a Tauren by the Undercity elevators?
Ever play a Tauren and not be able to walk inside Orc buildings?
Ever be on your Tauren and not be able to loot certain treasures because you can’t get close enough to the item to click it?
Unless you’ve played a lot as a Tauren, you have no idea how ridiculously unfriendly the game is toward large characters.
It’s also hilarious how most of the people asking for playable Ogres are posting on Alliance characters. Fine with me if Blizzard wastes the next Alliance race by giving them Ogres, though. Make them an Alliance race.