The problems are mainly three:
The first that what remained of the good old biggest communist party in Europe was blair-ized in the Democratic Party, and so a quite significant amount of formerly leftist old people until now voted it as an habit, even if nowadays it's almost fully neoliberal, cause everything to the left of it is perceived as a weak tentative and/or as a "gruppettari" autonomist thing, a.k.a. derived from the extra-parlamentarian near-terrorist left tradition, which was very conflictual with the Communist Party at the time.
The second is that autonomists are still quite strong, in particular in big cities, and squatted social centres are still very very frequented by people under 40 (grown with the communist party already dead so that they were all politically educated by the autonomists) and are almost hegemonic for everything regarding arts and cultural events, but they are disorganized by definition, so they have no political representation, are not united, and people in their political area often ends voting 5 star or not voting at all.
The third, and most important, problem is that the political space for a populist left, that as you said was occupied in France by Melenchon, in Spain by Podemos, in UK by Corbyn and so on, here was occupied by the 5-stars. Even if now their reactionary tendencies were revealed, at the beginning they seemed sort of leftist, like a mix between the Indignados movement and nordic Pirate Parties. Their main issues were direct democracy, public water, ecologism, hate for politicians (which were mainly represented at the time by Berlusconi), universal basic income, universal internet acces and so on. Actually, the most voted party by old Communist Party voters (with 35% of them) were the 5-stars. A lot of people, me included, thought that they were never going to make an alliance with the Lega.
Now that the 5-stars are revealing their inner fascism i hope most of their leftist voters will stop voting them (even if someone says that the voters will stick to it and become fascist too, but i hope this doesn't happen), and so does my party which is trying to take their voters in next elections: Potere al Popolo, a union of almost every movement and organization coming from the autonomist tradition not refusing contact with parlamentarian politics and party-like structures, some of the less useless remains of the old Communist Party, and a lot of smaller or territory-based organizations (Atheist Democracy, the Meridionalist party, Pirates' movement, no-TAV, protesters against U.S. military bases near their cities, and so on), with a very strong relationship with USB, a diffunded, fast-growing and pretty combative trade union.
pics related, i put in some /leftydrink/ meme not to derail too much xd
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