A lot of it has to do with the intensity of local civic/regional identity to any broader Italian one. In fact even in Italy today, there's a lot of Italians now who still currently identify themselves with their region (or city) first, and their national identity second.

In fact one could argue that the default state of Italian politics since the collapse of the empire is regionalism, and it took 19th century nationalism to make an alternative identity model for the peninsula attractive.

A lot of it also has to do with the fact that whenever any entity came closest to taking full or significant control of the peninsula, they were stopped by other powers who had a vested interest in either keeping the status quo, or keeping the area fractured. Geography also played a role, with the Apennine mountains acting as a physical communication deterrent to any long term conquest that experienced opposition.

You see this with the Byzantines and the Lombards. The Lombards and the Carolingians. The Ottonians and the Beneventans. The Holy Roman Empire and the Lombard League. The Milanese Visconti and the Tuscans. And so many other examples that escape me (because the high medieval era is not my specialty).

Basically, any powers that were capable of imposing any sort of political uniformity on Italy (after the Lombards), tended to be distant, thus when that distant power lapsed in interest, the cities of Italy reverted to their regional autonomy. And not until the ideology of nationalism, backed by the local Italian power of the Kingdom of Sardinia and uprisings elsewhere on the peninsula, could Italy be reunited.