Whatever major event has happened in history, from the Holocaust to the death of Elvis, you will always find a large group - worryingly large in some cases - of people who will either try and put a spin on history, or simply flat-out deny that the event took place.
I think it comes down to simple attention-seeking - if you go against the perceived wisdom about something, you are in a minority, and that gives you a sense of 'otherness' and 'specialness'.
That feeling makes you feel like part of a select, and superior, small group who actually 'know' what's going on, and are not fooled by the machinations of a secret cabal keeping us enslaved in ignorance.
It's complete bat-droppings of course, but it's part of the price we pay for living in a world where we prize and protect free speech.