The propensity for most people to believe in conspiracy theories often comes less from the fact that a person favors conspiracy as a more desirable explanation for things and more from the way in which they take in information and come to conclusions based on that information.
Some will be overly receptive to anything that smacks of conspiracy of any type, often bending the facts to support the sinister and to reject the benign. Others will reject anything that even sounds like a conspiracy theory, even if the information is true and is right before them. In the middle ranges are those who have come to believe that great conspiracies have happened and will continue to happen. But most will not automatically accept or reject the idea of a particular conspiracy theory.
The beauty of the term "conspiracy theory" lies in the word "theory". This means that inferences, hypotheticals and deductions have been derived from evidence or anecdote that leads to the idea that a conspiracy involving one or more people has been going on.
When the evidence or the anecdote is convincing, then more people will be open to the idea that a conspiracy exists.
A problem lies in the logical fallacies that get a good workout during most conspiracy theory sessions.
The "Appeal To Authority" fallacy is the most useful one, especially when the military or high levels of corporations or government are involved. Who is going to argue with an "Ex Manager", "Ex-soldier" or "Former CIA Operative" who was "there"? Many people accept statements from such individuals simply because of their title, claims of knowledge and standing, and not because there is proof or evidence to support their claims.
Another logical fallacy, that the rich, the powerful, the government and/or corporations are inherently corrupt and willing to do anything gives automatic power to any claim of conspiracy. When the perceived corruption combines with rising use of secrecy and enormous resources for suppressing or controlling release of information to the courts, then there is even greater inclination to believe in one conspiracy or another. Also, past, proved conspiracies greatly increase the acceptance rate of any additional, theoretical conspiracies.
Another aspect of conspiracy theory and its appeal lies in the levels of excitement, the preexisting attitudes toward the agency or group, and the drama that is involved. People love a good story and they love a scary story. Also, the complexity of the evidence and the conspiracy has a huge effect on willingness to accept a theory.
That a secretive group of the world's wealthiest and most powerful individuals plan to reduce the human population to a very tiny size is far more exciting and digestible than a complicated misappropriation of funds through a complex and boring series of high level financial transactions. When the audience cannot understand the mechanisms of the conspiracy, their ability to come to any conclusions will be severely limited. Predisposition will have to be strong for a person to believe something that they do not understand.
But when there are well presented visuals and information about a particular experimental radar array, plus an actual scientific expert who can explain why the radar array is a potential hazard, the public can understand and can accept a theory that some might be conspiring to misuse the array in ways that will cause wide scale harm to the planet. In this case, the conspiracy is to do potentially harmful science, just to find out what happens.
Finally, alleged conspiracies can be revealed through court cases, law enforcement investigations and political ploys. The tendency is to believe that, if the law is involved, then the parties have done some kind of wrong. But in many cases, the investigations do not pan out and there is not enough evidence to prove anything. Still, people will continue to believe that some conspiracy went on and that the parties are guilty, anyway...they just did not get caught.
In summary, predisposition to believe in conspiracies and logical fallacy are the primary reasons why people are inclined to believe in conspiracy theories. The fact that they are all theories and not proved fact does not get in the way. If the physical evidence and the persons giving anecdotal or analytical evidence are compelling and convincing, then an exciting new conspiracy is born! From there, it is all about word of mouth as the theory spreads throughout whole populations until perception management teams come into play.