One theory about the Renaissance is that it was caused in large part by the Black Death. The plague is said to have caused people to rethink the ways in which people looked at the world. It also caused social and cultural upheaval because of how many people died. For these reasons, Italy (and Europe in general) was more open to changes in philosophy and world view.
Culture usually spreads through trade. Trade allows people of different cultures to interact act and influence one another. Therefore Renaissance ideas spread partly because of Italy’s trade importance.
There are several theories to explain the driving force of the renaissance. I suspect there may be some truth in all of them, rather than one being correct and the others wrong.
1) The conquest of Moorish Spain. When the Conquistadors drove the Muslims from Al-Andalus (the Islamic name for Spain and Portugal) they captured the Islamic libraries of Cordoba, Grenada, AlHambra etc. These libraries contained not only advanced 'modern' Islamic scholarship, but they also held the texts of the ancient Greeks and Romans, which had been lost in the West for centuries. This refound old knowledge sparked a thirst among western powers to discover more about their illustrious, more-civilised past.
2) The fall of Constantinople in 1453. When the most advanced and magnificent city in Europe fell to The Ottomans, its Christian scholars were forced to flee into the 'wild'. They headed West, esp. to Italy, and they took their knowledge and books with them.
3) Last, but by no means least, the invention of the printing press, which was the medieval equivalent of the invention of the internet.
There are lots of other contributors, people, factors etc. But I believe that the main power behind the renaissance was a vast injection of new knowledge and a radical new means of distributing it.