Conspiracy As Causation

Conspiracies theories abound in our society for two specific reasons:

Some Conspiracies are Real
Events like the Teapot Dome Scandal, Watergate, and Iran-Contra really do happen - and each generation seems to have its own scandal - the Xers may even get a live one in the Plame-Rove Scandal. Sometimes, they're even successful such as the 1954 US-backed overthrow of the only democratically elected President of Guatemala, or the 1973 US-backed overthrow of the Chilean government.

Conspiracies as Creation Myth
Conspiracies theories provide meaning to random acts or events. I.e., it is much easier to believe that bad things are caused by bad people than it is to believe that bad things happen randomly. This is one of the central reasons that people JFK was assassinated by a cabal (be it government-, mafia, or Cuba/Soviet- planned).

These conspiracy theories share much in common with early creation myths - thunder is caused by Thor's hammer, life is painful and hard because of Eve's trangsression, Fall and Winter occur because of Demeter, etc. To paraphrase Fox Mulder's poster, "We want to believe."

We want to believe that bad people cause bad things because it is infinitely more comforting than the idea that bad things happen for no apparent reason. If bad things are caused, then the instigators can be found and punished. Or at least blamed, whether their true identity is known or not.