I don;t know why, but I have always thought that Nick Berg has something to do with this.
The bombshell we haven't seen is the footage Michael Moore took interviewing Berg before he was captured. He has buried it and won't tell anyone what was in it. I suspect it is because it showed Berg to be a proto-John Walker Lindh confessing support for the insurgents and how he was just there to "help" them a la Rachel Corrie. His interview would have added to 9/11's anti-Bush storyline if it had held, but the apple cart was upended when Berg was kidnapped and beheaded.
I'd really love to see that interview.