There's a number of things to address in this question; the first thing is the Veneto's steadfastness as a Christian Democracy stronghold, which is actually fairly consistent with present election patters, and the next is the strength of the Italian Communist Party in areas which where historic hotbeds of the Italian resistance.

The first, and easier, thing to address is the hard-left stance of the Emilia-Romagna. If you look at this map of German defensive lines during the second world war, you'll see they mostly cross Tuscany and the Emilia-Romagna; incidentally, the regions that would become most staunchly left-leaning in republican Italy. The simple fact that German military presence was strongest there not only meant that the people were most exposed to fighting which disrupted the normalcy of everyday life, but the high concentration of foreign military action also meant that corollary partisan activity was also more concentrated than in other parts of the country. With german and allied supply lines stretching far into opposite directions, the militarization of these regions coupled with the near-total absence of the Italian state meant that often, local antifascist filo-communist partizans were the only ones willing and able to defend the local populations, provide law and order, and even organize relief through supplies of essential items like foodstuffs and medical supplies. By war's end, not only would the memory of partizan activity be long-burned into the memory of local voters, but through local partizan militias the Italian Communist Party immediately had a groundwork for an elaborate network of local activists, organizers, and community leaders which would endure, in some form or other, from the 1940s to the modern day. This gave the Italian Communist Party a hold over the region much broader than its usual appeal limited to working-class neighborhoods and suburbs, typically in or near large cities (and university students, but that's a different dynamic). The fact that cities like Bologna and Florence would become important hubs for local industries, as were secondary cities like Modena and Livorno, certainly also helped.

The Veneto, on the other hand, is a different story. Typically, most Italian regions exhibited fairly consistent voting patterns: big cities which were contested between Communists and Christian Democrats depending on the size of the local Bourgeoisie (although the Socialists would be increasingly powerful after 1975); the Communists were immensely successful in working-class suburbs; and the Christian Democrats mopped up the remainder of small-town bourgeoisie and rural voters. This pattern is especially evident in northern regions, like Liguria, Piedmont, and Lombardy; however southern regions also could exhibit similar patterns (for example, the influence of the Communist Party in the Calabrese city of Catanzaro contributed to a weird and completely inefficient agreement splitting the regional capital between two cities). But the Veneto, very importantly, in spite of being a prosperous northern region, has the peculiar characteristic of having no large city where the Communist Party could be certain of gathering a solid block of consensus. Certainly, there were neighborhoods where Communist activism was very strong, like the working-class "Marghera" neighborhood of suburban Venice, as well as the vicinity of the University of Padua. However the absence of a concentrated agglomeration of working-class voters, as there would be in the suburbs of Turin, or in the suburbs of Milan (where the commuter town of Sesto San Giovanni would be dubbed "The Stalingrad of Italy") meant that left-wing voters were often drowned out by their neighbors. The lack of a single dominating urban center in the Veneto itself is an interesting topic, which has to do with the rapid depopulation and suburbanization of the region's chief city, which I discussed here.