Not sure if I have a good explanation, but the Right Side Rally Mirror was Optional equipment or came on certain packaged optional models.

My '71 SE only had the Left Side Mirror from the factory.

I bought a Factory Right side Mirror for my car in the late 1970's at a local Dodge Dealer. Mounted it myself by drilling holes and using the included mounting hardware. As best I recall, there was a reinforcing backing plate with this Right side mirror, but this Right Side mirror did not include a cable remote like the Left mirror from the factory installation.

Just thinking that the Left Mirror being required equipment had more effort put into design and planning than the Right side mirror.