Why are Superbirds worth so much?

A local guy here in town had 2 birds a yellow one and an orange one both 440 4bbl. auto cars. He sold the yellow one to a close "friend" or so he thought. The guy told he that was all he and his wife had wanted, 1 week later he sold it and dobled his money on the car. The story goes that when the people came to buy the yellow car , that a guy at a gas station pulled in and said would you like to buy another one? So they wound up buying the yellow one and a B-5 blue car at the same time. This was all in the late 80's. The original owner of the yellow and orange cars still has the orange car, and drives it every so often. Matt