The likely suspect is that the PC version has been murdered most foul by Games for Windows Live, which nears its last toxic-breath as the service prepares to shut down.
The trouble is not every GFWL title has been retrofitted to survive in a post-GFWL world full of sunshine and laughter. That means there'll be casualties, like Bulletstorm?
It does remain listed on EA's Origin store but it's MIA on Steam. Hopefully this is a temporary removal and someone has been tinkering with a new version. GFWL provided the multiplayer component to Bulletstorm. Codemasters' Fuel was another victim of the GFWL shut down, buthave also been updated like Rocksteady's Batman series.
People Can Fly's Bulletstorm delisted from Steam
21 March 2014 | By Simon Priest