what's with the Holocaust deniers?

Iran recently hosted a conference for Holocaust deniers. who would attend such a thing? David Duke is one. His racism is an easy motive for him. The same for the president of Iran. What is so powerful about racism? Hate definitely unites easily and blinds as well.
the Nizkor Project asks, "Given the evidence ... why do people deny the Holocaust?"

The wiki article on Holocaust Deniers states

Though they do not use the term revisionism, historians do continue to study and revise opinions on aspects of the Holocaust, though no reputable historian has challenged the basic scale and outlines of the event. In the words of historian Donald Niewyk from Southern Methodist University: "With the main features of the Holocaust clearly visible to all but the willfully blind, historians have turned their attention to aspects of the story for which the evidence is incomplete or ambiguous. These are not minor matters by any means, but turn on such issues as Hitler's role in the event, Jewish responses to persecution, and reactions by onlookers both inside and outside Nazi-controlled Europe."

as with most conspiracy theories, the effort to set up the conspiracy would be greater than the actual event occurring. from the same Wiki article

Historical proof for the Holocaust, such as maps and documents or buildings claimed to be gas chambers, have been deliberately falsified, misinterpreted, misrepresented, or reconstructed. For example, documents contain grammatical errors which only non-native German speakers would make; photographs of generic death and destruction are labeled Nazi atrocities.

another helpful resource is called answers to the 66 questions of the HD's