What are the factors responsible for the spread of renaissance in Europe?
The two primary factors responsible for the Renaissance were the increasing wealth of the Italian city-states and the fall of Constantinople in 1453.
The city of Venice had grown quite wealthy after the Fourth Crusade. It had funded the Crusade and in return received tremendous wealth from the Crusaders following the sack of Constantinople. Merchants used this wealth to build a vast trading system whereby they traded European wool in Asia for silk, jewels, spices, etc. Other city-states shared in this prosperity. Among those gaining wealth in the trade were the DeMedici family who believed that they should share their fortune with the people of Florence. They commissioned works of art, statuary, etc. as a means to endow Florence with many artistic works.
With the fall of Constantinople, many scholars from that city moved West where they were employed as clerics and tutors. They brought with them the knowledge acquired over the years in that city. (Constantinople had never labored under the close-minded doctrine of scholasticism that had pervaded Western Europe.) With the introduction of this new knowledge to Western Europe, scholars there attained an interest in reading the Bible and other works on antiquity in the original languages. This gave rise to humanism, a substantial element of the Renaissance.
The influence of Constantinople can be seen in many works of art of the period which are quite classical.