Seeing as how there is a great deal of photographic proof, a tremendous number of people who survived and told their stories afterward, and you can still visit the actual sites where it happened, some of which were left mostly as they were; this is a very interesting question. I think it goes back to what I said in my answer to your other question about Hitler, and it's simply the fact that, once a person develops a deep-seated belief, based solely on hatred or other strong feelings, there is no amount of evidence nor logical thinking that can change their opinion. I'm not going to say that every Holocaust denier is a rabid anti-semite, but well, 100% of the high profile ones are just that, so, likely a majority of the time, it's these people's burning racism that feeds the conspiracy theory.

They don't want to have to deal with the guilt.

If you know about separation anxiety, it relates to that in that it is a separation from life denial thing, get what I mean? People today participate in relishing being wrong anymore, don't they?

The Holocaust is a sympathy campaign maintained by the power of suggestion. Photos of personal belongings don't prove that former owners were exterminated in gas chambers - but by telling you that they were, you immediately believe it - because your emotional response has been triggered. The same goes for the emaciated bodies. It is claimed that they were starved to death, but nothing is ever said about the disease epidemics that were rampant in the camps that took huge tolls on the prisoner population. The "Final Solution" was created in January of 1942 and yet, Hitler was first accused of exterminating Jews on Aug 6, 1933 - 9 years before the so-called "plan" to do so was written. A newspaper article appeared in the New York Times on Jan 8, 1945, that told of "6 million Jews dead". It wasn't until January 27, 1945, that Auschwitz was liberated - how could this "6 million" number be known before Auschwitz was liberated? The "6 million" number and "extermination" go back as far as the turn of the century in newspaper propaganda designed to raise money and sympathy for the Jews. The stories ran many times between 1900 and 1939, and by the time Hitler was first accused of exterminating Jews, threats of Jewish extermination were already well absorbed in the mind of the public. The association with Hitler resonated with people and Hitler became the evilest man in the world.