I never gave much thought, or researched why the belts were mounted inboard. Guessing...perhaps it was easier to mount the retractor in the middle than in the cramped area next to the seats by the doors? Just a guess?
I do know that the reason they were mounted on the doors in the early cars was a government thing. The idea was that they should be left buckled, and a person would lift the belts to get in the car. Lots of cars had that arrangement and those awful motorized shoulder belts too to comply with the rules at the time. Canada didn't have that law.