read about the congress of 1921
that plus the most influential socialists didnt support armed resistance, for some reason, and the communists were led by Bordiga, who was excessively careful, really not wanting a civil war that would cost the lives of thousands of workers and bring what he saw as "state capitalism" in place of the monarchy
however there was armed resistance, the mayor leftist parties really just had to proclaim their support, there were dozens of syndicalist militias, republican socialist and even communist war veterans, grupe di difesa proletaria, Ardity del popolo, not to mention the anarchists, and as said before, people like d'Annunzio, who really didnt care who they fight as long as they can do it for freedom, chauvinism, and the inspiration to write more futurist poetry
olnly real problem was, aside the disunion after the 1921 congress, the fact that the police, the army, practically all the state aparatus, the church, the industrialists, the big land owners, foreing investors if there were any, banks, nobility, and the monarchy was leaning on the side of the blackshirts, basically waiting for them to sort things out, because noone wanted to really go and mess with difesa proletaria groups or get into firefights with armed workers on strike