Hi there! I've written on the topic at length, most recently here, although there's certainly room for more discussion and I'm more than happy to answer additional questions.

Dense urbanization, and political institutions built atop that urbanization, were defining characteristic of the Late Roman Empire's Italian heartland. While the Imperial superstructure decayed and eventually mutated, these urban institutions found a way to persevere. Perhaps encumbered by the legal and social baggage of the Empire they had to replace, once Roman institutions definitively evaporated with the fall of Theodoric's Ravennate monarchy, the two post-Roman kingdoms of Italy (Lombard and Carolingian) demonstrated an enormous potential for regional instability. The Italian characteristics of florid cities with prosperous hinterlands allowed political factions with small geographic power bases to easily challenge the new political order, while the new political order even in times of tranquility could never assert itself over the urban institutions. In few other places in Europe (and nowhere as extensively as Italy, although honorable mentions abound) did religious and political authority so definitively converge onto the old Basilicas, and seldom was the local senatorial class in a condition as those in Italy were to maintain the trappings of collegiate rule that has so characterized government in the Empire. Even in the wake of Justinian's devastating "reconquest" of the Peninsula, the entry of a foreign warrior class was largely a story of assimilation rather than domination.

Conquest, ensuing ill-fitting legal constructs appropriating Roman heritage, and cynical acceptance of that superstructure as long as its practicalities remained vague, was all made possible by the success of the urban political communities in Italy.