Brakes for 70 duster small bp

I know I'm new here but........I'm just curious as to why its such a pain to change to larger bp rotor's from small bp. I have original disc brakes with small bp....wheel selection sucks. It seems someone would make a rotor that fits the disc brake spindle with the upgraded larger bp without having to go to a drum spindle where one change ends up being another change. It seems it would be a whole lot easier to find one from another vehicle ( ford or Chevy if need be) with the same size bearings that would make it a lot easier.... or an aftermarket one. Just think there would be a more simple approach to this.