Does anyone know?

The forward position IMO looks better and gives the driver a more comfortable viewing angle. My eyes have a hard time focusing with the mirror moved back. I think Ford may have been trying to lessen the blind spot created with the mirror so close to the A pillar.

 

I like them snugged in closer to the A pillar a bit for styling reasons. I think the viewing angle is a bit better that way too, but have yet to test this with some actual driving. With the mirrors in the original 72 position, I always remembered leaning a bit to get a better look.

I did mock them up in several positions further forward, and it is possible to go too far, in which case, when viewing them from a 3/4 front view, they start looking like Dumbo Ears. I found I like them best if you position them with the front screw approx. 8 and 3/4 inches back from the front door gap. It also is roughly 5 inches back from the lowest point of the drip rail.

If other models moved the mirrors back at the same time, maybe there was some hokey federal regulation or corporate rule that came into play, "Mirror must not be more than xx inches from the driver's position ..." or something like that. It is still hard to believe that passenger side mirrors were considered optional at that time.