The real staying power of this story is that it is extremely difficult to prove that something didn't happen.
True Holocaust Revisionism wouldn't have to prove that the Holocaust didn't happen. It would merely assess the wealth of evidence available and propose an alternative historical narrative described by that evidence, and make an argument for that narrative being a superior interpretation of all the known evidence.
Holocaust Denial doesn't do that. It can't do that, since there is no plausible narrative that would even remotely challenge the conclusion that Nazi Germany attempted the genocide of European Jewry and succeeded in murdering something like 6 million of them by various means.
What Holocaust Denial does is deny. It denies the evidence. It claims that dozens of Nazis only admitted their crime because they were tortured. It claims that numerous Jews who witnessed key murdering modalities are liars, simply because the Germans treated them badly. It ignores or maligns Germans, Poles, Ukrainians and others who testified to the mass murder of Jews. It declares a mass of documents fake, or mindlessly maintains that German words don't mean what every German historian, indeed every objective German, knows they mean. It lies about whether mass graves have been found at some death camp sites, as it has lied about a half dozen contemporaneous aerial photographs taken of the Birkenau site in 1944-1945.
It has on occasion attempted to guild its empty message through pseudo-scholarly articles which are filled with lies and distortions of the evidence, or which focus on a few pieces while ignoring other, sometimes more important corroborating evidence. As the dishonesty has been exposed over the years, it often moves on to new dishonest claims, which then must be painstakingly debunked.
The only Holocaust Revisionism being undertaken is that done by historians who utilize new evidence, or accept stronger interpretations of existing evidence, to continually refine the history of the Holocaust. That's the way the revision of history works. It doesn't work by a small band of cranks and Jew haters starting out by wishing to whitewash Nazi crimes, and then merely saying there is no evidence.