I think part of the reason it isn't remembered is that up until this invasion Hitler and Stalin were allies and the progressive left in this country supported Hitler. The progressives don't like to be reminded of this fact.
And there's the shameful story about how union members sabotaged the armaments they were employed to build because the arms would be used against then Soviet ally Germany.
The left opposed Hitler right up until the moment Hitler and Stalin signed their pact. Then their masters in Moscow told them to support Hitler and amazingly enough, they did. No thought, just follow orders from Comintern.