The bad economy.
You can get a Wii for $250 or the lowest Xbox 360 for $200 OR you can get a PS3 for $400. People really don't have the extra cash to spend right now and are not going to go out of their way to get a PS3 now when the Wii is legendary and the Xbox 360 seems to have most of the "must have" games this season.
Sony really has dug themselves in a hole and its not one they will likely get out of soon, if ever.