I think its a fairly recent development. I don't remember a bunch of late models at rod runs/car shows back in the 70s. But then the cutoff for a lot of car shows was 1948. I think the NSRA, Goodguys and others have caused the infiltration of Camaros, Mustangs and late models into the car shows. I guess it takes a lot of work to go down to the dealer and buy a new pony car, drive through a car wash then head to the car show and spend a hour polishing it.