One reason is that people on the left are more inclined to disagree with each other, which isn’t necessarily a bad thing, and accepted as part of the political process. You figure things out by voicing differing opinions and reaching some kind of middle ground. There’s room for disagreement on the left.

People on the right (fascists in America) are more likely to do what they are told by their “superiors” and fall in line. Differing opinions, or lifestyles, are not readily tolerated and you will be excommunicated if you push the accepted boundaries too much.

You can say a lot of negative things about fascism, and you should, but fascists tend to have a lot of solidarity as long as they toe the line. That is, until the critical breaking point, which is usually everyone that isn’t a fascist realizing they actually have to deal with the fascist threat, at which point the fascists start collapsing because you have to be stupid to be fascist so fascism can’t really stand up against a united front against it.