Carl Trueman, writing a few years ago:
Conspiracy theories have an aesthetic appeal: they make us feel more important in the grand scheme of things than we are. If someone is going to all this trouble to con us into believing in something, then we have to be worth conning; and the impotence we all feel in the face of massive impersonal bureaucracies and economies driven not by democratic institutions so much as multinational corporations is not really the result of our intrinsic smallness and insignificance so much of our potential power which needs to be smothered. Such views play to our vanity; and, to be brutally frank, the kind of virtual solitary vice which so much solipsistic internet activity represents.
Conspiracy theories don’t hold up, though. Nobody is that competent and powerful to pull them off. Even giant bureaucracies are made up of lots of small, incompetent units fighting petty turf wars, a fragmentation which undermine the possibility of the kind of co-ordinated efforts required to pull off, say, the fabrication of the Holocaust. History, humanly speaking, is a tale of incompetence and thoughtlessness, not of elaborate and sophisticated cabals. Evil, catastrophic evil, is not exceptional and brilliant; it is humdrum and banal; it does not involve thinking too much; it involves thinking too little.
For more, see Trueman’s excellent book, Histories and Fallacies: Problems Faced in the Writing of History, especially his chapter on Holocaust denial.