Epic Games' Cliff Bleszinski revealed that co-op play was fully implemented in Bulletstorm but it was removed after the play-testing showed that it ruined the game experience.
"It was in there at one point and it actually worked, but we found that the game shifted from being this kind of puzzle-shooter into essentially this downhill skiing simulator, where people were seeing how fast they could get to the bottom of the mountain," Bleszinski told The Telegraph. "Really the game just broke down, and it was a situation where people would race through the game, ignoring the vistas and they wouldn't set up as many skillshots."
"So when we made the decision to go straight campaign, it gave us the chance to put in one-offs that could only be controlled by one player, like the giant robotic dinosaur, where you don't have to worry about Player 2, or slow motion one-offs that could only occur in a single-player game."
It is worth noting that Bulletstorm does have multiplayer support, including "Anarchy" mode where up to 4 players team together to fight increasingly difficult waves of enemies.