Probably because people with a normal level of human empathy and moral consciousness realize the following:
- That Nazis kill people for who they are, which is something one cannot choose. Nazis exterminated 6 million Jews, and millions of disabled people, gypsies, Slavs, and other “undesirables.” By contrast, communists killed people for what they did. Of course there are some exceptions. One must consider the historical context of pre-WWII Russia, along with Operation Trust, the planned coup, and the mindset after the horrors of the Russian Civil War.
- Communist is against anti-Semitism. Nazism is highly anti-Semitic.
- Nazis performed painful, brutalizing, and cruel human experiments on adults and children. The “Angel of Death,” Dr. Mengele, performed horrifying experiments on twin children. 
- Nazi racism was only matched (and inspired) by U.S. Southern racism, which was some of the most virulent and brutal racism in the world. In fact, some black POW’s from the Korean War chose to move to Red China instead of return “home” to the American South, which was dominated by the KKK, lynchings, police brutality, and Segregation.  The Nazi miscegenation laws were actually modeled after U.S. miscegenation laws. How the Nazis Used Jim Crow Laws as the Model for Their Race Laws
- Generalplan Ost was the German plan for the starvation, shooting, and mass extermination of millions of Slavs, Jews, and others in the Soviet Union after the Germans conquered it.
Clarence Adams, a black American POW, chose to stay in Red China after being captured during the Korean War as a soldier for the U.S. military. The disgusting and dehumanizing racism he faced at home and in the military convinced him to try something new. So he did. He met a Chinese woman, got married, and had children. He lived there until the Cultural Revolution, at which time he moved back to the U.S.