I think it's simply that they couldn't include all possible species at launch, and they had a vote to decide on which ones to put in, which would be easiest to design different facial features for, outfits for, etc.. I am pretty sure Togruta will make it in at some point, especially considering 1) they're already in the game and 2) they're very popular.
Well, not necessarily. WoW launched with the absolutely most dismal customization I've ever seen, and they've only marginally improved it with allowing people to change hair and transmogrify gear in more recent times. 25 man raids back before Cataclysm was like an army of clones. Yes, you can be some really different looking races now, but good luck distinguishing yourself from everyone else that plays that same race.
I guess I should have been more specific when I said customization. I was talking about species diversity. In WoW at launch, you could play any of the traditional high fantasy races, plus cows and undead. Then they added draenei, wargen, and now pandas. That's the kind of diversity I'm talking about. They didn't just offer recolored humanoids.