I would note that Italy also had a an influx of different peoples in the wake of the fall of the Western Roman Empire. Obviously there were goths, but also Lombards that moved into the north.

I would also point out that trade made many aristocratic families and their corresponding cities crazy wealthy without the corresponding land and population. Their long links to the Mediterranean allowed them to develop monopolies with Eastern trade as the gateway to the Catholic west. This is probably why mercenaries were so popular - money, but not a lot of foot soldiers. This might be how they could stay divided as long as they did, and not conquered by someone else. It also made them rivals and emphasized some of their local culture as a matter of pride.