The most common theory as to why only one door lock, on the passenger side, is that vehicles back then parked along the streets, and being narrow, it was not safe to exit the drivers door, so the logic is that it was expected drivers would slide across and exit and enter the passenger side door. Hence the one lock. Perhaps that and the savings of leaving off one door lock would mean for GM.
But I have heard that some even earlier vehicles did not even have a driver side door. So the door minus an outside lock may just be in the evolution of the vehicle.
