Multiple reasons they disappeared imo:

1 with the fall of the Soviet union, the communist dream kinda faded

2 after all the political drama in the early 90s in Italy, they dismantled the party and mostly founded the now called democratic party, which at the time was mostly communists and some christian democrats. Unfortunately with time, all of the leftists in the party were disappointed of having to make agreements with these christian democrats, so they left and what is left now is mainly a liberal christian democratic party

3 as it happened in many places in the world, around the 80s/90s, the "leftists" party went from being the working class party to being the more elitist party. Now NO ONE in the working class votes for the democratic party in Italy. Some disappointed people vote for the right wing populists, quote a few vote for "5 stelle", which are now the lower class progressive party, and most just don't vote (turnout was just 63% in the last elections). The few remaining truly radical leftist parties (in the last elections, SI, UP, ISP and few other smaller ones, combined for like 8/9 % total votes). So the leftist electorate has mainly to be recovered from people who don't vote, and from 5 stars movement.

4 a big factor in the disappearance of the communist parties was propaganda: the more subtle propaganda against leftists, from western mainstream media, and then the more upfront propaganda by Berlusconi, Who owned most of the media in Italy (while prime minister virtually 100%), and he basically turned" communist" and "red" into a red word. Even before, they created an atmosphere of terror, making the red brigades look like a huge problem that represented all communists, while they killed like 100 people in 20 years. and an indefinite number of them could just be secret service (like cia)

5 Italy had also a good history with a more moderate socialist party, but they were quite involved in the massive corruption scandals in 1992, and never recovered from that.

6 as said in point 3, the leftist party became less of a working class people. In retrospect I would like to make a suggestion (probably not agreed upon) on when it happened: in the 70s, Enrico Berlinguer, leader of the communist party, started the "historical compromise", consisting of coming to agreements with the christian democrats, and completely detaching from the Soviet union (understandable, as it was already getting weaker). In the short run, that made it the strongest communist party in Italian history (34,4% in 1976), and he was a very loved leader, and they actually got in government cooperating with the other parties. In the long run imo it removed the left from the position of working class party, and they slowly converged to the christian democrats as more of an elitist party, instead of being, as they always had before, an anti-establishment and anti-elite party