There is a very interesting article on why conspiracism is so pervasive and popular, despite its obvious stupidity, here at TCS Daily
Interestingly, the author, Edward Feser, a conservative professor of philosophy, gives an explanation that is virtually the opposite of mine. I see these "theories" (which aren't real theories at all, as I've said) as offering all the comforts and advantages of irrationality, while he seems to see them as the product of an over-reliance on -- reason! He points out the the intellectual posture of the conspiracist is inevitably that of the lone skeptic, casting aside "the official story" and thus overthrowing the power of authority, to which the masses have uncritically submitted. The governing myth, in his view, is that of the Enlightenment: truth is on the side of the lone intellect, while custom, tradition, and authority represent nothing but illusion.
As a matter of history, I think Feser's Enlightenment-run-amok theory is plainly wrong. Modern conspiracism begins with the Abbe Agustin Baruel's writings about the Bavarian Illuminati. The Illuminati -- the bad guys in this conspiracist yarn -- are Enlightenment extremists, free-thinking foes of both king and church who sought to establish a new regime of freedom and reason on the ruins of all traditional institutions. The first thing the Illuminati did, according to Baruel, was to cause the French Revolution. (How? I guess they pulled some of those "strings" you've heard about.) The arch conspiracy theorist of my own lifetime, Robert Welch, the founder of the John Birch Society, believed the the Illuminati were the ones at the bottom of all the other conspiracies. The Commies were just one part of it.
Anyway, Baruel's conspiracy "theories" were apparently the product of a profoundly reactionary hatred of the Enlightenment, not the reverse.
I have to admit, though, that Feser's view does explain something that mine so far does not: that conspiracism tends to have an anti-authoritarian slant. The conspiracist is usually, like Huck Finn's Pap, "agin the guvmint." Or in some cases, the Church. (Da Vinci Code, anyone?)
So I have to add something to my explanation. But what? I'll have to think about that.