u/Yodatsracist, an expert on comparative religion, has discussed the multitude of approaches that scholars have used to compare European and American religiosity in a great answer here, although I do think that in the five years since that answer has been posted Norris and Inglehart's thesis (the most recent one discussed) has gained more traction, even though I personally like the "national identity" argument (I can corroborate that the history of Italian Unification in the 19th century is as much as anything else the story of the bourgeoisie, characterized by nationalism, secularism, and liberalism, butting heads with more conservative institutions: monarchial governments (not just limited to the Austrian Empire), the Catholic Church, and certain groups of the aristocracy).

In the specific context of Italy, I also wrote about the complicated relationship between the Italian Communist Party and the Soviet Union here and some considerations about historic voting patterns in Italy here. Although each European country had its own drivers pushing voters to the left, in Italy a mix of urbanization, activism, and the consequences of the second world war led to the creation of the largest non-ruling Communist Party in Europe.