Because games needs visual updates and it's just the way things are. If the game was never updated visually and we still had a vanilla quality graphics across the board it would be very bad indeed. This can only be a good thing. That people want to criticize this is beyond me, and they often don't understand how games are made and think this will somehow delay the expansion by months. If anything, it would be strange if they did not update the models at this point in time.
To say that time spent developing something that isn't "gameplay" isn't worth it must come frome the mouth of someone who is really out of touch with what a video game is, it's akin to people who think that spending money IRL on leisure and entertainement is a waste. I for one care about how my characters look, I'm not an RPer, I'm just a regular PvEr and yes i like looking at my chars, tinker with their appearance to a minute level (Even though that is not really possible as things are now). I take great care in making them and modifying them. Visuals matters, and if you think this is unimportant because most of the time you are looking at their ass 20 yards away you will be proven wrong I think.
And I have to laugh at people pointing out that it isn't near the level of SWTOR, Rift, GW2 or whatever, WoW is from 2004 and it has intentionally had a cartoony style since then, hell even before that. You are just pointing out the obvious. No one ever expected the graphics of WoW to approach those, if so a complete overhaul of the game would have to be done. As an avid SWTOR player myself it is fully possible to be reasonable and not be so bloody polemic, WoW is WoW and ToR is ToR.
Personally I can't wait for the new models to be playable and it will be 10x as exciting once we have the full specter of customisations available, even if they don't introduce sub-races. The only thing that I miss from other games is that we should be able to choose at least a body type like skinny, normal, fat, bulky etc. But no big deal really.