It looks like the license for Ghostbusters: The Video Game, released back in 2009 for last-generation consoles and PC, has ran out. The game has disappeared from the Steam Store - although is still available from some other online retailers.
You can still play the title if you own it but until - or even, if - the license is extended, it won't be available for sale. The developers, Terminal Reality - who went on to develop The Walking Dead: Survival Instinct - went defunct and shut down back in December 2013 - meaning its all up to publisher Atari.
The title has received praise and attention since its release as "the third Ghostbusters movie." As described by Ghostbusters star and creator, Dan Aykroyd - who also starred in, and wrote the video game. Each of the original actors from the movie returned to reprise their roles with Max von Sydow returning to voice Vigo the Carpathian.
Ghostbusters: The Video Game was also recently added to the Xbox One's Backwards Compatibility - so fans can return to New York City on Xbox if they own a copy.