The question isn’t “why doesn’t the Republican party enact rules to stop Nazi racists from running on their platform?” The question is “why do all the Nazi racists identify with the Republican party?” You can’t dismiss this as just one lone crank when he’s part of an ongoing trend.
I like to think that if every white supremacist candidate ran as a Democrat and every white supremacist organization in the country endorsed the Democratic candidate for President then there would be some bare minimum of soul-searching within the party’s leadership to figure out what the hell was going on.
Basically, anyone who denies the Holocaust is really just itching for another chance to give it a try.
The whole basis of the denial is that the Jews are so powerful and magical that they masterminded the whole thing, from the Nuremberg Laws to Auschwitz, faking mountains of German paperwork, faking photos and films, and secretly smuggling millions out of occupied Europe through war zones. In other words “the Jews secretly control the world”, it’s a way to sow fear and hatred, and ultimately spread the idea that “something must be done about them.”