It's not that Trump is a fascist. Far from it. He's a 1980s Democrat. It's that they just can't think of a worse thing to say. It's the worst insult they can think of, so they use it freely.

You have to realize just how much Trump damaged leftist psyches by being elected. At a stroke, everything they knew about the world was proved wrong. This led to crippling cognitive dissonance, as they realized that Hillary was a criminal warmonger who belongs in prison and how could I have voted for her? Bad, bad, bad feelings. They have to get it out somehow. And screaming is what they chose. The worst thing they can think to scream is "Fascist". So they scream all the time, trying to feel better.

The idea that maybe they were complete assholes and that was the problem was rejected. They aren't going to change. It is the American people who were wrong.