This may have been explained in story in a past mortal kombat, but either I don't remember or I never knew. What happened to Shang Tsung's ability to morph into other Kombatants?
It was in the DA storyline...ther ewas not enough RAM in the land of Mortal Kombat to allow him to swap models/skins during a fight. Even though he does morph into kung lao at the begging of DA, to kill Liu Kang.