The problem is that it provides further evidence that we are there for the correct reasons. Also, if the terrorists that did it are truly Al Queda, it again gives further justification for the POTUS GWOT and his decision to go into Iraq in the first place.
Anything that proves that the administration was right is going to be downplayed by the media while anything that puts in it a bad light is going to shown repeatedly to convince the uninformed and apathetic portions of the public to vote for that other guy from Massachusetts.