Th work required for the task of perpetuating a monstrous lie is far greater than that needed to simply let the truth shine through.
Big lies are easier to put out there and maintained, because they are generally a lot easier to remember the details of, and harder to refute due to their repetition.
Compare it to the quote from Josef Stalin: "The death of one man is a shame; the death of a hundred men is a tragedy; The death of a million men is a statistic."