The issue with the 14" wheels and factory disc brakes is the contours of the inner wheel, between the mounting pad and the back half of the rim. As noted, things CAN get close between the wheel and caliper.
Initially, easier to use 15" wheels than to find a vendor with the necessary size/capabilities needed for the weight (actual and loaded weight) of a C-body and larger. Not to forget that the rotors cool from the inside diameter outward as the wheels turn. Having the outer rim so close to the rotor might hinder that in a mountainous drive in a loaded C-body wagon with a pleasure boat hooked to it, possibly?
Chevy put 14" wheels with factory disc brakes on the first gen Camaros, BUT they used special 14" wheels, which is where I learned of the "inner wheel contours" situation. As disc brakes were seen to be "the future of things", some universal wheel designs became available a year or so later.
Just some recollections,
CBODY67