A 1970 Fury III vert with E86, manual brakes & no power steering? Sounds rare enough.
To the original question:
I suppose the regular 4 door Fusies were in most cases just abandoned and/or rusted away in the late seventies. They weren't fashionable then. Those were times when a four year old car was considered old. Only the rare birds - verts, Sport Furys, and the occasional loaded 300 or Imp - were cherished, and garaged even back in the day. Then, in many cases the garage queens were eventually just forgotten there, as the original owners got too old to drive them. So now all we have left are those rare birds.