This question takes in the best part of a millennium and a half of West European history, and I was aiming to find 3-4 earlier answers that might address parts of it.
However, it turns out u/AlviseFalier has written an epic answer to a very similar question: It seems odd that the Italian peninsula could give rise to the Roman Empire and then largely resist political unification until 1870. How have historians explained Italy's extended political fracture after the Western Empire fell?