Why are Superbirds worth so much?

I saw a nice big poster on a Chebby mans office wall one day. It was a 70 Superbird parked on the salt flats and was stock except for the salt tires and flush wheel covers.

The caption underneath was "Worlds fastest stock car at 216 MPH". I'm betting it had the overdrive 4 speed and I'm pretty sure it held that record into the mid 80's. Makes ya wonder.8)