The thing about 9/11 is that we want a better explanation than a group of "3rd World terrorists" pulled one over on the "mightiest nation on the planet." We want a "truth" that reinforces our view that the US is the strongest, best nation on the planet, and has no need to fear anyone else. The only people strong enough to take us down are "evil us."
Sorry. The truth is that Al Queda and Osama Bin Laden aren't "3rd World Terrorists." They're just plain terrorists, educated in more than few cases by our own military, intellegence and colleges, trained and hardened by decades of war. They think long term, and they plan meticulously. 9/11 wasn't the result of a few weeks of planning, or a "hey, let's try this." I'd have to look up -- but some of the perpetrators had been in the US for years. They had studied and planned, working around our vulnerabilities. They exploited our ignorance, our complacency, and our reactions to prior hijackings.
That's important to understand. Until 9/11, the routine response to a hijacking situation was to sit tight, and wait for the plane to land. Then, the options were either wait 'em out or storm the plane. The hijackers on 9/11 exploited that: They took control of the planes, and on 3 out of the 4, the passengers didn't expect anything but the usual scenario.
We all aided and abetted the terrorists. We assumed that "terrorism happened somewhere else", to other people. We carefully avoided labeling terrorist activities that had happened here as terrorism. We refused to acknowledge how terrorists work, and we still refuse to acknowledge that we DO NOT make friends in a lot of the ways we try to help other countries and people. Especially when we stomp roughshod over their cultures and try to make them like us. Or when we assume that we're the best and our ways are the best. (Just look at some of the heat on this forum across national lines... And that's with fairly reasonable folks.)
There was a conspiracy involved in 9/11; in fact, I'll say there were two. The first, clearly and obviously, was the terrorists. That one involved at least 20 people, probably many more. The other? That was all of us who conspired to live out a myth without accepting the reality of terrorism, and of the people out there who don't like us.