Italo (and H35), why Treviso and Parma? These are relatively small population zones no? Is there any rugby presence in the bigger cities like Rome, Milan, Turin? I understand the national team are based in Rome.
They play at Stadio Olimpico basically because it's still Italy's showpiece stadium. Nowt to do with the geography of rugby support. Rugby in Italy is pretty much wholly based in the north. There is very very little professional rugby played south of, say, Bologna. I'm in Tuscany, 90 minutes south of Florence, there are a couple of small amateur clubs in the area. Football (soccer) is God.
The northern roots can be traced back to early in the 20th century when industrialisation kicked off in the north. It brought an influx of English engineers, workers, etc, who brought rugby with them. Cricket, too, for that matter. Cricket is still played in the north believe it or not. But only in the north. In addition to the English input, the jobs attracted a lot of workers from just across the border in southern France, their rugby stronghold. And so it went.
Rugby will always be a minority sport here. Basketball draws bigger TV audiences. Nick Mallett, to his credit, when Italian coach, stressed that rugby had to be less northern-centric to grow. He drove the establishment of Italy-wide academies. And, for his troubles, was squeezed out by internal rugby politics. Viva Italia. The young talent coming through from the academies over the last few years is largely down to Mallett.