Mostly war-weariness.
Not to mention at the time radical socialism was new and hardly anyone imagined how problematic it'll be in the future.
More than that, the urban working classes, minimally to a very substantial extent and maximally almost universally (the difference being as much as anything different historians’ opinions),
supported radical socialism and were highly hostile to the traditional Catholic or Eastern Orthodox authoritarian systems that ruled most countries bordering the USSR on the west.
Although in most of interwar Europe this urban proletariat was a minority reviled by almost every other sector of the population – the landowners, the peasantry, the middle classes, and most of all the churches and the political class – technological change was consistently increasing Europe’s comparative disadvantage in agriculture and thus increasing the power of this left-wing industrial proletariat. Thus, containing its latent power was an essential goal for most of Eastern and Southern Europe during the interwar period, at which they were politically successful. However, if they had invested in a war to defeat the USSR, working class anger – latent under actual traditionalist authoritarian systems – might have become much more open and “swing” sectors of society (the middle classes, the peasantry and the intelligentsia) might have abandoned their hostility towards the urban proletariat. If they did, the urban proletariat would not have been a powerless minority, but could have joined in a majority to
spread Marxism.