Because 90,000 Italian citizens live there. If the city of Pisa did not exist, none of them would have a home to go home to, and every single one of them would quickly become a burden on everybody in the rest of the country.
The city of Pisa was at the beginning a simple but important Italian seaport. With its growth, so did its religious buildings. Its fame and power grew gradually over the years, as the people of Pisa were involved in various military conflicts and trade agreements.
