Why does Italy remain fragmented in the 12th and 15th centuries?

In the 12th century, Italy consisted of cities with the status of communes. Each commune was headed by an elected consul. The communes were not dependent on the feudal lords. The communes differed among themselves: some were stronger, some were weaker, but they were all republics. There was always a struggle between them. The communes were not interested in unification, they did not want the country to be headed by one ruler, since it was not profitable for any republic.