My guess it was parked in the late 70's and avoided the relentless salt covered roads in the 80's and 90's, saving it from certain rusty quarters.
My '67 was last licensed in '91 but it still has some rusty quarters. It was a one owner car (grandma of course), Grandma lent it to her niece to drive, the niece and her boyfriend put dents in it and wrecked the 283, and it has been parked ever since. In fact her name is on the original title and sales paper work.
'67 Impala Fastback - 2012 Project
"Buy Them, Hide Them, Restore Them!"