Because small-minded, empty-headed buffoons enjoy feeling as though they possess knowledge that other people don't. It makes them feel special. Holocaust deniers are notoriously thick. Not to mention extremely xenophobic and racist. They're well-known for being gullible, the kinds of people who watch poorly done YouTube videos and attempt to cite them as a valid source. The wide majority of them are just worthless, bitter, lonely failures. How many have ever accomplished anything in life? How many could sustain a conversation with a female without forking over some money? The Holocaust is one of the most well-documented events in modern history. The evidence that it happened is overwhelming and irrefutable. The underlying question when it comes to any conspiracy theory is "Why?" Why would the British, Americans, Russians and other Allied troops lie about it? Why would the Germans themselves lie? Why would the people who were liberated from camps concoct stories? It's ridiculous.