Like any other manufacturer, the "Bean Counters" have their snouts stuck in many places. I have a GM friend who told me about a retainer that "Someone" in the front office determined they could do without on several vehicle lines. The .02¢ savings per vehicle would seem petty to us commoners. The "Counters" figured that same 02¢ multiplied by just one-days production at several GM plants ensured their job security, and as they say, the rest is history.
The mirror issue on your '72 could have been something similar or a supplier issue (I believe the new word is Supply Chain Issues). There was an engineering/supplier problem that delayed the availability of the automatic seat back release on the 71 Mustang until 1/71. Vehicles equipped with the Convenience Group option and built prior to 1/71 would reflect a partial credit on the window sticker for the missing automatic seat back release since it was part of a group option. So with a process that involves thousands of parts and constant engineering and running changes, these sorts of things were expected. Since these racing-style mirrors were also used on many different Ford and Mercury vehicle lines, it could have been a supplier capacity issue. These mirrors were pre-painted by the vendor, so there was definitely a higher cost involved than the chrome mirror. It could have been a decision by our "Bean Counters" to make them an extra cost option on all models except the Mach 1 and Grande'.
It's hard to point a finger at who and why a decision was made 50 years ago. Any engineers from that era that are still around probably get a good chuckle when thinking about us trying to sort through all results of their decisions these many years later!!