Another possible comparison is Albinoleffe, though Cittadella aren’t the result of a merger.

And don’t get me wrong, I like seeing Cittadella in B, but come on, 1000 visitors average? What are they? Chievo II?

There is also an anti-Milano money and power element to it. People who had moved north in order to find work and found themselves being discriminated against in all aspects of daily life because of their origins found it very hard to support either of the two Milano teams (for the context in which this happened, you should watch Rocco and his Three Brothers, which has nothing to do with football, but is a great film). Juve were the only alternative among the big three. For Southerners who had to leave Italy to find work, it was helpful for them to “find” a club whose results they could actually follow and which their fellow workers in Wolfsburg or Clermont-Ferrand had actually heard of. Juve fulfilled both criteria.

The extremely strong sense of local identity that one still has in the South has always made it difficult for a “Southern SuperClub” to arise. Napoli have come closest, but the vast majority of Avellinese would still rather cut their hands off than support anything from Napoli, and that dynamic is replayed throughout the region (Palermo-Catania, Bari-Lecce, Napoli-Salernitana, etc).

Giro, I root for two. Bari and Juventus.

Juve used to buy all their young talent from southern teams (big fish in a Serie B pool) and a lot of fans just continued to root for their heroes that made the trip north. The Northern teams (From Roma up) would have their own youth teams, buy from other Serie A clubs (which are mostly in the northern half) or buy foreigners.