Mirror glass is cheap, 'high resolution', 'high dynamic range', tolerant of getting dirty. A high quality, small camera and matching screen that are as good seems expensive, so would probably be limited to high end vehicles at the minute.
If my wing mirror gets broken, I can replace it myself for the price of a round of drinks and the housing is easy to bodge with duct tape to get me home.
Reversing the question - given that it is not allowed, why should a manufacturer spend money lobbying to get this requirement changed?
Matt