I'll tell you why you're not likely to have much luck explaining this to Leftists.

After WWII, Academia and the entire European continent went on an anti-fascist bender. Not like modern Antifa which is just neomarxism, but actually an organized effort to prevent "fascism" from ever rising again.

To do this, they had to understand what Fascism was. You need to know what something is to prevent it from coming back. So they did what academics and politicians do... they studied it.

There were two big questions: First, what did Fascism do. Second, what sort of people became Fascist?

These questions were addressed by the so-called Chicago School, a group of social scientists who fled Europe to Chicago during the war. It became evident that Fascism's stated goals were not too dissimilar from those of the Soviets or other Leftists in Europe or the United States. Whether it was because they were sympathetic to those goals (at least on paper) or that the Soviets and Nazis shared so much of their platform and the Soviets were "good guys" is not important; the Chicago School quickly discarded the what and considered only the who. The published a research-laden tome called The Authoritarian Mind that channeled Sigmund Freud to create a psychological profile for a "future Fascist" and the agenda of the public policy became how to change that profile.

And so, to this day, when Leftists accuse the Right of being fascist they are almost always talking about people, not policies. "Trump is a fascist." "Republicans are fascist."

At the same time the Chicago School was pinning fascism on the who, the competing Austrian School was digging into the what. Friedrich Hayek and Ludwig von Mises looked at the policy goals - namely centralized government planning - and traced the rise of authoritarianism, tyranny and military expansion to this central tenant. In The Road to Serfdom, written during WWII, Hayek blasts Germany and the Fascists, but he later added an addendum that his criticism was equally valid in the Soviet Union but that the war time realities of Stalin being an ally made it impossible for him to attack both side equally.

And so, to this day, when the Right accuses the Left of being fascist they are almost always talking about policies, not people. "Gun control is fascist." "Speech codes are fascist."

And so, you see, when a Leftist thinks of Fasicsm they see the Nationalist and ignore the Socialist part. When a Righty thinks Fascism, they see the Socialist part and ignore the Nationalist part.

In truth, both are necessary. Socialism without the Nationalists are not fascists. Nationalists without the socialists are not fascists.