It's far less frightening for some people to think that the nigh-on-omnipotent US Government was responsible for something like 9/11, than it is for a handful of dedicated people to have done it.

If only the US had had a sustained campaign of high profile terrorism carried out against it for several decades, as most of Europe has (ETA, the IRA, Red brigades, etc, etc) then there would not be this refusal to believe that terrorist could do it...

There is no arguing with the Non-believers. It doesn't matter if they've decided that the Earth is 6000 years old, or there were no planes, or whatever it is. They have faith, so no evidence or explanation will convince them otherwise.