The reason "he" was made the focus was that the real culprit of 9-11 was Saddam Hussein and we could not go after him at the time due to our military unreadiness, thanks to der Schlickmeister. So Afghanistan was a response to our immediate outrage after the attack and our desire to "Blow something up in return"...as well as provide our troops with a crash course in war.
Of course, I could be wrong...