Hmmm... that's actually a different question than your title. The "why" could just as well be asked of people who think we never went to the moon, that the earth is actually flat, and such. That is an inquiry into human psychology, not history. (Still...a pretty interesting inquiry!)
As to if its reasonable to question the factuality of the Holocaust, I cant determine what is reasonable for other people. From my perspective, the evidence supporting the Holocaust is absolutely undeniable. Photographic evidence (NSFL), the physical existence of the camps, primary accounts of the camps which range from camp gaurds to camp liberators to imprisoned Jews suggest that the genocide happened, and futher supports the idea that it existed on a massive scale. Further, the Nuremberg trials were heavily based on primary evidence, and concluded that the Holocaust existed. And these examples are but a drop in the bucket of a massive collection of stories, confessions, and testimonies concerning the extermination of the Jewish people.
For me, it is simply impossible to deny the Holocaust.
There are a lot of reasons why a person might deny the holocaust: ignorance, contrarianism, or a general conspiratorial attitude. However, the most infamous Holocaust deniers generally do so from a politically biased perspective. The Holocaust had tremendous political and social implications for many Jews, including the creation of a Jewish homeland in the Levant. Anti-semites, Arab nationalists, and Anti-westerners have rejected the Holocaust that the might sabotage the legitimacy of Israel, as well as justify their own extreme rhetoric.
This isnt to say that there isnt interesting and important questions which need to be asked of the Holocaust. Many historians have, and continue to, explore the question of "What did x person know, and when?" or "How did the Holocaust evolve as national policy?" Holocaust historians continue to explore these questions, and ask hard questions to try and explain why such a sad and traumatic event happened.
As a final note, Ive really focused on the Jewish Holocaust. The Russian genocide, which goes hand-in-hand with the Jewish Holocaust, is really very similar. In fact, many Nazi's were more willing to admit to what happened in Russia, given the climate of Post-War politics concerning the Eastern Bloc. But at the same time, AFAIK there are no serious deniers of the Russian Holocaust, especially because the Russians rally around it as a nationalistic point.
Good examinations of Holocaust denial are Richard Evan's Lying about Hitler and Deborah Lipstadt's Denying the Holocaust.
