I just disagree the KH stuff is superior. Why were they replaced? Why didn't the bigger cars get a version? Bottom line is they SUCKED. They were replaced with a better, newer, more modern version.
The bigger cars got the BUDD brake system in '67 and '68, also with 4 pistons per caliper.
They worked quite nice on my '67 Newport... right untill the unobtanium seals started leaking because of the rusty cylinders in the calipers.
The factory most likely found a cheaper to produce brake system and went with that later on.