The broad masses of a nation are always more easily corrupted in the deeper strata of their emotional nature than consciously or voluntarily; and thus in the primitive simplicity of their minds they more readily fall victims to the big lie than the small lie, since they themselves often tell small lies in little matters but would be ashamed to resort to large-scale falsehoods.
It would never come into their heads to fabricate colossal untruths, and they would not believe that others could have the impudence to distort the truth so infamously. Even though the facts which prove this to be so, may be brought clearly to their minds, they will still doubt and waver and will continue to think that there may be some other explanation.”
Adolf Hitler, Mein Kampf
The concept of the Big Lie, described in the above quote, explains why so many people are willing to believe that small lies/conspiracies exist (such as a snooker club being deliberately destroyed as insurance fraud), but are unwilling to believe that big lies/conspiracies exist (such as the World Trade Center being deliberately destroyed as a pretext for the war on terror).
They can conceive of small lies/conspiracies, because they can imagine themselves doing it, or something similar, if they were desperate, and not quite so morally upstanding. In many cases, they will accept the existence of small lies/conspiracies even with no evidence at all, relying purely on their intuition about human nature and the motives at play.
They cannot conceive of big lies/conspiracies, because they would personally never be evil/bold/shameless enough to tell and maintain a big lie, and they assume the same is true of everyone. They cannot conceive of any big lie/conspiracy being possible to maintain, so they assert that no big lie/conspiracy exists, without considering any evidence. The effect is so powerful, they will go to great lengths to avoid looking at any evidence that contradicts their faith that Big Lies are impossible.
The Big Lie effect is part of the explanation of how evil conspiracies can be and are successfully maintained. The moral goodness of the masses is what makes them vulnerable to the Big Lie effect.