Saying somebody is a fascist is not the same as saying they're a Nazi. Fascism has a few competing definitions, including Umberto Eco's 14 points and "palingenetic ultranationalism". Both of these definitions match Trump very well. The latter essentially means national rebirth by returning to the old ways, of which "make america great again" is a perfect example.
Also, the classic alt-right threat of "if you keep calling us fascists then I guess we'll become fascists" is... Basically just an admission of being a fascist, or at least proto-fascist? It's not the "gotcha" you think it is.
[edit] also, voting for Trump doesn't make you a fascist. Plenty of different interest groups voted for him and most of them, I would say, aren't fascistic.