GM has a bad habit of letting the bean counters run wild after a car has been introduced. Usually this makes the introductory year an orphan as the parts are changed to cut costs in materials, or in the labor to produce the car.
It happened with the 1967 Camaro, and the 1968 Nova and to a large extend the 1965 Impala (they even changed the dies so that many sheet metal body parts do not interchange). When these cars were new it wasn't an issue, because federal law required that GM maintain parts for at least seven years after a car was sold. After that junk yards still had good used parts.
But when it comes to the reproduction part it is a matter of supply and demand. B-bodies are not in demand in general; and the 1965 has less demand than the 1966-'67 production years thanks to a certain TV show that I have never even seen, and few Punisher movies.