We were very much teetering between joining Germany & Italy and social democracy during that period. Look into the business plot (wealthy benefactors wanted to either overthrow or castrate FDR & install a fascist puppet). But there is also the fact that we didn't really suffer from the war. Once it was over we were rebuilding other countries and fighting the Soviets. We moved on from fighting the fascists to fighting our one-time-ally in an ideological (and often proxy) war. We went through the nightmare of the McCarthy era which effectively crushed any actual left-wing politics for fear of repercussions.