It's way more than just making new wireframe models and skinning, and then designing animations for movement, emotes, combat, etc. That's probably one of the smaller parts of the task.
The part most everyone seems to NOT realize, is that suppose they finish all the models.
Great. Now they have to move on to rebuilding, remodeling, and reskinning ALL EXISTING GEAR.
Unless you really want a brand spanking new human model, with armor clipping through the body everywhere, with boots that are now ankle high, or out of proportion, or look like 4 polygons over your nice new 3091380985 polygon count model.
Every piece of gear for every race/gender model would need to at the very least have its scale played with to make it fit right to not look totally bad.
That's pretty easy for most vanilla cloth gear, since it seems like a lot of that was just flat skin on the model itself. But any of the complex gear would need quite a bit of tuning to fit any new model they come up with.
So in summation; It's a way bigger task than folks seem to think. It's not something you could round up the art team to work on on weekends and finish in a few months. It's a pretty large undertaking.
Just look at things from Cataclysm like Stormwind. Stormwinds buildings are a static object. Not much real motion needed, banners and flags and such I'd expect are already made. Redoing that, to make it repaired since Deathwing is gone now would be way way WAY easier compared to the task of new character models & animations. Yet they've not done it, because it is still quite a bit of work for little return.
So try to have some patience. It is a lot of work.