Because the ideology of Communism is internationalist instead of nationalist, anti-imperialist, socially modernist (ie. feminist, etc.) rather than traditional and chauvinistic, materialistic, and interprets history through economics/class rather than ethnicity.

While the NSDAP and other fascists were anti-capitalist and anti-conservative to varying degrees (atleast in rhetoric), they saw the capitalist bourgeoisie and aristocratic classes as allies and necessary for society, as long as they were subservient to the State and Party.