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.