This is a rare case of the novel being more outlandish than the film.
In the book, Goldfinger hijacks a BOAC Constellation, which was a giant piston-engine airliner that was fast approaching obsolescence even in 1957 when the novel was written. By the time the film appeared three years later, its BOAC equivalent was the Boing 707. However, the hijacking of a commercial flight makes no sense (Fleming's ideas were often nonsensical), so Eon gave Auric his own Lockheed Jetstar, which was the world's first bizjet. While it makes no narrative sense that the authorities didn't impound it, I imagine Lockheed allowed Eon access to one for promotional reasons, which would explain its two appearances. As CJB says, one less set to build as well.