As much as I appreciate the ammeter, I don't know why they seem to have been preferred over voltmeters well into the 70s. Even the aftermarket underdash gage sets came usually came with ammeters, not voltmeters. Maybe accurate voltmeters in that range were more expensive??
Could be. Voltmeters aren't built much different. It's not like the manufacturers had to wait for the computer before they figured out how to make one.