It was presented in a manner convenient for the conservative parts of the military, but among the republicans and other people rebelling at home against the war were socialists, an ideology which were perceived to be Jewish in origins.
Marx, the father of scientific socialism, was a Jew, son of many famous rabbis, and other educated German Jews were on the Left at the time, Ferdinand Lassalle for example, the founder of the German Socialist Party, or Leon Trotsky in Russia, the Marxist revolutionary and theorist. And many Jews held prominent posts and were very visible in public life during the Weimar Republic.
These and many other factors, besides convenience, made the 'Marxists' primarily a Jewish movement in National Socialist optics.