It seems clear to me that high level officials in the Bush administration have had a vested interest in allowing the terrorism threat to America to fester. After all, 9/11 worked out so well for Dick Cheney and his neocon gang in giving them the pretext they needed to initiate their invasion of Iraq. If they just let the al Qaeda situation fester for long enough, then maybe they'll get lucky and score again - this time getting the pretext they want to bullshit us into a war with Iran.
U.S. Aborted Raid on Qaeda Chiefs in Pakistan in '05 (The New York Times)
How else can you explain the unwillingness to secure the capture of Osama bin Laden in Tora Bora back in 2001?
US let bin Laden slip away, inquiry told (The Sydney Morning Herald)