The film itself was rubbish.
Like I said to a buddy of mine that claimed that he was disappointed with how much Twilight sucked, "did this REALLY surprise you?" Maybe I've got a sixth sense when it comes to movies, or maybe I just pay way too much attention to previews. The last movie that actually shocked me by being terrible was Kung Pow (I expected it to be a stupid movie, but stupid in a funny way, like Dumb n Dumber... It was indeed stupid, but very unfunny)...
A third option I just thought of that may play a slight role, (but I doubt I'm as easily tricked by myself as would be required for this to be the full cause) is that maybe I prejudge a movie from previews, and anything judged as crap will be considered crap unless it is an absolute masterpiece...
On an unrelated note, if you haven't seen Tenacious D and the Pick of Destiny, you should. If you haven't seen it since theaters, you should buy it now. If you own it, you should be placing it in your DVD player before the end of this sentence. If by some odd chance you were already watching it while reading this thread/reply, turn it up. :P