Go tell someone who survived Auschwitz that the Holocaust didn't happen. Go tell someone whose entire family was obliterated. See what kind of response you get.
Above and beyond this, there are some more general ideological problems involved with the idea that the Holocaust didn't happen:
They have little to do with disputes about the number of people who were killed. During Stalin's regime, an estimated 30 million people were murdered, but not with the same brutal, bone-chilling efficiency exemplified by the Third Reich. (I am by no means condoning Stalin's actions in saying this, but only trying to make a larger point.)
One problem is that this idea trivializes the horrific nature of genocide--the systematic dehumanization and extermination of a group of people based on one shared characteristic of identity. Pick any characteristic you want.
Remember that Gypsies, homosexuals, Catholics, mental patients, academics, and political dissidents were also slaughtered by the Nazi regime.
Remember also, that genocide has occurred since the Holocaust. I refer you to Sins of the World.
"Those who cannot remember the past are condemned to repeat it."--George Santayana