The Three Reasons
The first reason is that the Roman Catholic Church everything they could to stop the rise of a powerful kingdom in Italy. They were afraid that if a strong ruler united Italy, the same ruler would be able to control the pope and the church.
The second reason is that the city-states that were in Italy were all about equal strength. They fought many wars and often captured territory from others, but no one was able to defeat all the others.
The final reason is that the city-states were very wealthy. They could build big armies and hire mercenaries, or someone that fights in their army. They also loaned money to the kings of Europe. The kings left the city-states alone so they could borrow money in the future.
Wealth
The city-states became wealthy through trade and not just art. Italy's location was a good spot for trade. The Italian peninsula meant that most of the cities had docks and ports where merchant ships docked. Geography wasn't the only thing though. Several events led to trade becoming important in the city-states. First, the Crusades brought Italian merchants into contact with Arab merchants. Second, the rise pf the Mongol Empire united almost all of Asia into one vast network. The Silk Road was protected by the Mongols and the Mongols also encouraged trade. This made it cheaper and easier to carry goods from China and India to Muslim and Byzantine cities. As more and more silk and spices were shipped from Asia, the price of the goods fell. More Europeans could afford it and had a bigger demand for it. As a result, business for Italian merchants continued to grow.
