This is probably my main reason for doubting the conspiracy theories; when I see the federal government in action it does not seem competent enough to pull off a complex conspiracy.
My main reason for doubting it is that when you see a perfectly plausible explanation for something captured on video, like oh, say several tons of airplane and explosive liquid crashing into a building, and then later that building falling down, and then you discover that the planes were hijacked and flown into the buildings by fanatical enemies of the country, it becomes hopelessly ridiculous to believe that the buildings fell from some OTHER hidden means, and that the airplanes crashing into them weren't the REAL reason they fell, but simply an elaborate cover for the the real reason, that seem superfluous on its face, 'cause why wouldn't you just blame the actual method of the buildings falling on the patsies in the first place?
So basically, my main reason for doubting a government conspiracy is that the lack of evidence on the one hand and the overwhelming evidence on the other is only surpassed by the total, utter, crazy ridiclified nature of the theory in the first place.