Most people who believe this bunk don't understand what the mercury retrograde really is. They think it's something serious, when it is really just an optical illusion in the sky. Mercury appears to be going backwards because of our perspective from here on earth, and the relative movements of the other planets. For a sky-watcher, it's interesting. For the superstitious person, it's momentous. Like zodiac signs in astrology, it guides their choices and gives meaning to events.
People who embrace "new age" spirituality are the most likely to believe in the negative effects of a mercury retrograde. They also tend to believe in karma and reincarnation. These beliefs are indicative of the overall ignorance of our populace. People think that because they have shed institutionalized religion that their new superstition must be better. Too many people seek explanations for what they experience, and then latch onto them without further consideration.
How is reincarnation any more plausible than heaven and hell? Are we capable of living with "I don't know" as our answer? Is intellectual laziness really our future?