Italy was in the 15th century the wealthiest region in Europe. Its merchant class, clergy, aristocracy had money to pay for artists and could afford to sponsor various large cultural, architectural, and artistic projects of that time. Italy did not experienced significant warfare as did Western and Central Europe due outbreak of 100 Years War that devastated medieval Europe. This helped to have more capital surplus than rest of Europe.
2) Italy has the largest heritage of the Roman civilization in Europe. Europeans were always fascinated in Roman past, but they had hard time to connect with it (for example due lack of eduction). However, the Roman past was present everywhere. More the Italians had uncovered about the lost civilization, the more they were fascinated by it. It generated more interest about Roman culture, writing, art, architecture, and way of life. Europeans in the medieval and renaissance era were not aware of any Greek contribution toward the Roman civilization. This was discovered much later, in the 18th century, when the historians proved that the Greek culture predates Roman.
3) Relative decline of France. France was the largest and the most powerful medieval state in Europe. It gave the world a Gothic art which become the imprint of the medieval era. French dynasty, society, and monastic orders (Cistercians, Premonstratensians) were radiating its culture to the rest of Europe. But the 100 Years War devastated much of France, even Paris. The French dynasty did not provided cultural influence; and this void was filed with Italian clergy and papacy (it also moved from southern France back to Italy). Popes and the clergy influenced the rest of Italy well before the outbreak of Reformation. Renaissance predates Reformation by 100 years.
4) The level of urbanization due more advanced economy. Italian cities were the largest in Europe, and were banking centers of the medieval world. Outside Italy, only Paris could compare to the size of Milan, Venice, Florence, Pisa, and Napoli. The cities had very diversified economy and complex social classes similar to modern Europe than medieval times. Large merchant class was literate and was harder to control by the church hierarchy. The modern society was far more secular and more open-minded about various topics like nudity. Renaissance was a break-point from medieval piety because of its secularization.
5) Political structure. Because Italy was fragmented and lacked significant center of gravity, each urban center had own cultural influence. The larger cities were also competing for the best artists of their times.
