They don't want you to be able to kill Sigmar most likely. Though who knows, you might see a model for him in a later campaign.
Edit: I saw the model. I can see what they were going for but they just didn't get there. He looks fat, the pose lets him down tremendously. The hammer isn't the centrepiece either, which it should be. I get the shape of the wings and whatnot, but they could have preserved that and still had a much better pose of the body.