Because they're BOTH left wing ideologies.  What you see is factional in-fighting.  "Nazis" (NSDAP: National SOCIALIST German WORKERS Party) were socialists.  The "Communists", (also left wing) whom the American left has dearly loved for decades, were responsible for hundreds of millions of dead, making Hitler look like a Piker!  Looking at the political spectrum, think of it as a circle....not a straight line.  Hitler, being a socialist, borrowed many ideas from early 20th Century American Progressives.  Do some reading on THAT evil ideology: All white Utopian Society, elimination of mentally & physically impaired, elimination of "Negroes, Jews, Slavs.."etc.  The White Supremacists, KKK, are spawns of the democrat party.  The left is very good at trying to control the narrative, and put those groups on the "right" of the spectrum.  But anyone knowing history knows THATS a HUGE pile of horse puckey.