It is not that Holocaust Deniers repeat an assertion on the face of authenticated and comparatively recent historical facts. They count on repeating their assertions through the years as the past goes further back into history.
Once the people who lived through the 1940s die off. Holocaust deniers are free to twist facts even more.
They also leap into the next atrocity and deny it ever happened such as the genocide of muslims in the former Yugoslavia.
I have even seen arguments that no black slaves were forced to stay with their white masters in America during the civil war era. They were free to leave whenever they wanted. An argument that is gaining ground with the alt right.