Common Dreams has a review by Peter Phillips of David Ray Griffin's book The 9/11 Commission Report: Omissions and Distortions.

In particular Phillips identifies the psychological resistance many people have to hearing, knowing or discovering the truth about 9/11.

Two quotes from the article:

We all remember, as young children, scary locations that created deep fears. We might imagine monsters in the closet, dangers in a nighttime backyard, and creepy people in some abandoned house down the street. As we get older we build up the courage to open the closet, or walk out into the backyard to smell the night air. As adults there are still dark closets in our socio-cultural consciousness that make it impossible to even consider the possibility of the truthfulness of certain ideas.

...

The idea that someone in the Government of the United States contributed support to such a horrific attack is inconceivable to many. It is a threshold concept that is so frightening that it brings up a state of mind akin to complete unbelievability.

However, most people's experience is as Griffin describes in the introduction to his first book on the subject: he assumed the truth of the "official account" of 9/11 until he started looking at the evidence for himself.

That's what you've got to do.