The short answer is propaganda deriving from the one major example of successful socialism, the USSR, and the large amounts of post-hoc disinformation fueled by cold war hysteria.

It's very easy to construct a narrative that both were evil and godless empires around the events that transpired in the 30s between the M-R Pact and Brest-Litvosk, and the photos of Nazi and Soviet troops celebrating together don't exactly help to dispel the mythology surrounding these events. The truth is far more complex, but it's easier to accept that your ideology is the better, more humane one than to listen to "revisionist" history which counters a narrative built and honed over nearly a century.