No reason was provided for the removal of People Can Fly’s stylistic 2011 shooter. Intrepid folks believe its delisting is related to rumors that the entire Games for Windows Live client will be discontinued this July. Microsoft had already shut down its marketplace features last year.
The game is still available via Origin, but if that version is also removed, consider the above rumor pretty much confirmed. The folks at SlashGear speculate that, as was the case with games like Bioshock 2 and Fallout 3, the game could resurface via a Steamworks replacement.
If you already had Bulletstorm in your Steam library, fret not; its delisting shouldn’t affect your ability to play it.