it is supposed to be due to them not being able to see inside of your car to see if you have a gun/knife or something when they are pulling you over and ask you to get out of the car, its a hazard for their own safety of the officer.
I would be fine with this reasoning if not for the fact that vans, SUVs and limos are exempt. Because, y'know, only law-abiding citizens drive those vehicles and would never reach for a gun.