Bulletstorm developer interview on the Telegraph talks with "Clifford" Bleszinski, asking the artist formerly known as CliffyB some questions about Bulletstorm as Epic prepares to launch the first-person shooter they co-developed with People Can Fly (thanks Computer and Video Games). One question asks why there's no campaign co-op, and Clifford explains that they found that it made the game into a "downhill skiing simulator," saying:

First off, 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. 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.