A major factor:

For much of the first millennium the Church and what scholars there were, were bogged down in the Platonic view that what we see in the outward world is like distorted shadows on a wall (Plato's Allegory of the Cave), and the Truth is to be found by looking inward—the contemplation of the True Essence of Things. The Church translated the realm of True Essences onto "Heaven" and inward contemplation into meditation and prayer (guided, of course, by the Scriptures). Knowledge came through God's revelation.

In the wake of the Crusades, many presumed lost manuscripts and scrolls were brought back from the Middle East, among them, many of the writings of Aristotle. Aristotle's world view was diametrically opposed to Plato's. Look outward, examine what you see, and think about it. Not just random speculations, but logically (and Aristotle essentially invented logic, writing a substantial treatise on the subject which is still considered valid today). The mathematics of thought.

Thomas Aquinas tried to use Aristotle's principles of logic to prove the existence of God. His understanding of Aristotle's logic was not too good, and all of his arguments contained fallacies (but that doesn't stop some modern theologians from still using them). But the effort got others interested in the use of logic—in combination with Aristotle's other principle: observation. Many of these folks did observe, took what they saw and thought about it logically as Aristotle said, came to tentative conclusions (theories), then went to take another look. The scientific method.

Galileo. Copernicus. Soon whole bunches of others.

At the same time, painters developed perspective, and learned about anatomy (Leonard da Vinci, Michelangelo), because they looked, and painted what they saw rather than trying to work from some inward vision. A millennium and a half before, the Greeks, judging from their statuary, were quite good at this, but it seems to have been forgotten during the Middle Ages.

Not the whole story, but that was the core of it.

People started looking for themselves and thinking about it rather than simply accepting what they were told by priests and aristocrats.

Don Firth