Now, granted, the Holocaust affected more than just the Jewish, but the table above points out the amount of total deaths being almost 6 million. Include other "purged" groups, and the toll raises to well above six milion.
The main reasons one would deny the Holocaust is if one cannot believe the extent or human cruelty, or is a Nazi symphasizer. There are other, small reasons, but all are insignifigant. People cannot believe the extent of human cruelty, while every day they do anything to push their way past others. Let's face it. Hitler was a nut. He killed people. Humans kill other humans, and often, for the same reasons the beasts in the fields kill people. We want power and position, and we'll kill for it. If you sincerely believe the Nazi's weren't that bad, you need to wake up. If you believe that the ultimate goal of the Nazi's was good, go take a cold shower. In a gas chamber. Feel what they felt. The Nazi's kept good enough documentation, the facilities are still around (or proof of them is), and there is enough evidence. If you don't think there's enough evidence, ask any Holocaust survivor or any veteran who liberated the concentration camp. There are literally thousands of photographs and many more written documents to prove this. Heck, read The Hiding Place, that's a great book.
If you still aren't convinced, watch some documentaries on the Holocaust. The only deniers are those who are too dunderheaded to know what the word "holocaust" means (Holocaust literally means a blazing fire, of intense destructive capacities). In Greek, holocaust is formed from the words for "completely" and "burnt". And they put those together, obviously.