The obvious inability of apparently rational Americans to conclude that jihad exists, and that some Muslims express themselves through jihad, is a mystery. For some, the link between jihad and the Koran is not conclusive, since, as far as they are concerned, every religion except perhaps Buddhism enjoins its followers to smite the wicked and the unbelievers. So since every religion does this, it is for them impossible to conclude a relationship between Islam, jihad, and terrorist explosions.
Many of these people are in good faith.
They are employing a burden of proof towards the relationship of Islam to jihadist activity that they do not employ in most other circumstances. If they maintained the same burden of proof towards the dangers of crossing a street through heavy fast moving traffic, for example, they would tend to be killed. But when it comes to distributed patterns over time and space, failure to perceive patterns is both easier and serves other, more comforting purposes.
What is the cause of the blockage?
I think that the cause of the blockage for many is that, if they concluded such a relationship existed, namely that as Islam:jihad::Communism: class warfare, they would feel obliged to take much more extreme action than they would like. For many, the existence of such a link would present problems of conscience because they would feel the next step might be the surrender of valuable civil liberties, or worse, mass arrests, deportations, even massacres. So denying the causal link is the basis of continuing to feel comfortable with discomforting facts.
In other words I do not believe that the jihad deniers are unaware of the facts; they resist drawing a picture to themselves of the general relationship among the facts because to do so would force them to conclusions they feel uncomfortable with.
But as the article by Stella Paul in the American Thinker said, we are simultaneously being asked to see something, but denied the legitimacy to say in public what we see, which is jihad.