I'm currently doing a little bit of study into the history of communism, so I'll hit on a few points. The NSDAP was, by definition, a right wing völkisch-nationalist party that emphasized German national identity. Communism, by definition, is an international movement. Whether or not the communist regimes that exist(ed) were truly internationalist is a different iss(ue) This is exacerbated by the fact that "politically, the Nazis saw Communists as their most bitter enemies", largely due to their internationalist focus and the fact that Karl Marx’s father was the son of a rabbi who converted to Christianity (by force) only a year before Marx was born. (Brown, 138; Priestland, 23) Add in the fact that Communism was most represented at the time by the USSR and, according to Nazi ideology, Russians and Slavs are subhuman.

As the German economy grew worse and worse, "support for both Communists and the radical right increased". (Priestland, 188). The left was divided between the Communists and the Social Democrats and the right, while not unified entirely, was still dominated to a certain degree by the Nazis. Add in the fact that the army was more likely to support a conservative movement such as the Nazis meant that by 1932, there was little that the left could do to stop the rise of the Nazis. As Spielvogel and Redles note, many on the left "simply hoped and believed that the Nazis would somehow fall and they would survive". (Spielvogel and Redles, 74) I'm sure someone else can go into far greater detail, but that's the cliff notes.

 

Sources

Brown, Archie. The Rise and Fall of Communism

Priestland, David. The Red Flag: A History of Communism

Spielvogel, Jackson and David Redles. Hitler and Nazi Germany: A History