Their reason for this is clear, if you listen to the foreign minister of Iran:
"If the official version of the Holocaust is thrown into doubt, then the identity and nature of Israel will be thrown into doubt," said Mottaki, whose ministry put together the conference.Translation: They are looking for an excuse to sponsor additional terrorism by groups with a goal that amounts to nothing less than genocide.
Among the attendees is David Duke, who calls it a triumph for free speech. It isn't. It is a gross misuse of free speech.
You know, one JDAM properly targeted could make the world a much nicer place. Holocaust denial is something that is a strong indicator that a person is a waste of oxygen. Other countries have made it a crime to deny the Holocaust. The United States should follow suit.