People associate fascism with the Nazis, and the Nazis with mass murder. Disprove the Holocaust, and fascism loses this stigma. There are other, deeper reasons, I believe, that underlie the revisionist movement, having to do with the larger movements of pseudoscience and pseudohistory. Reason and rationality, as skeptics know too well, are under attack on all fronts. No claim, no matter how absurd, is immune from belief by someone or some group.
History exists only in the minds of historians.
The past is constructed by historians, much as sculptors construct figures out of marble. Historians can no more purge themselves of bias than anyone else, including physicists and biologists. There is no complete causal structure of contingent events in the past. Historians construct a causal structure in their minds out of the available documentation. Historians' job is to present this constructed past not as it actually happened but as it might have happened in one interpretation only.