How so? It organized itself in the form of a party that got voted to power; it stayed in power through a democratic mandate. It just happened that said mandate was upheld through a campaign of intimidation and brutal elimination - just as with communism in the USSR.
On the practical level, Nazism was opportunism incarnate. Had Hitler been prompted to emigrate from Vienna to the United States rather than to Germany, he would no doubt have attempted to enlist the support of the black population once his initial fantasy of a more bourgeois revolution failed, much like he rode on agendas appealing to the economically downtrodden in Germany.
Plus, everybody already knew enough to fear communism, even if much of the knowledge was outdated by half a century and fell short of predicting what was actually happening inside the Soviet Union. Nazism didn't have this sort of historical ballast, despite choosing "socialism" as one of its buzzwords; OTOH, anything with "national" in its name would feel at the same time a bit threatening (because most nations are not your own) but also a bit soothing and familiar (because nationalism is easy to associate with).