This is an extremely controversial topic. Ian Morris's Why the West Rules - For Now ascribes it to the "advantages of backwardness". Basically Western Europe profited from the technological advances of other civilizations, and then was able to capitalize on them in ways the originators didn't.

A specific example would be the sail. It was developed in China, but the rough Atlantic seas, and the loss of the overland trade route from Europe to China after the Crusades lead to the age of sail. The trade expedition where Columbus "discovered" America was the product of those developments. The Chinese didn't have the same risk/reward from losing the overland trade route, and they therefore had less of an incentive to send massive trade fleets east.

I highly recommend the book, for such a dry topic it's really accessible.

To kind of boil this down from a Chinese perspective, China was always truly a Middle Kingdom to the rulers and people. Starting from Silk Road days, goods from Asia, especially China and India, were constantly being traded West. With China, this was not only confined to resources, but also meant technology (water clocks, wheelbarrows, the Zhuge Nu a 4th century BCE repeating crossbow, many others). So once a real established route between China and Europe was established, there was a clamor for these items and ideas.

To China, Europe wasn't a well known entity. And what they did know of them didn't make them care to enquire that much more: after the fall of the Western Roman Empire, these Europeans that the Chinese encountered were deemed to be fairly uncivilized. The Chinese felt their own position to be superior to the Europeans (as they had every right to be, with cities far larger and more complicated than European cities during the Middle Ages). It's not a what that changes this, but a who: Chinghis Khan coming around in the 1200s with the Mongols.

Long story short, the Mongols wrecked first Tibet and then both the Chinese states at the time: the "Great Jin" (大金) in the North, themselves a steppe people and controlling what is modern day Beijing, and the "Southern Song" part of the Song Dynasty, controlling what is more inner-mainland and South China. The damage was catastrophic both in terms of life and infrastructure. The garrisons, farmers, and average citizens were slaughtered and the countryside and cities were burned and looted. Much the same was done later in the Islamic world, perhaps the only close seconds to China at the time in terms of knowledge and power. Europe was relatively unharmed (Russia and Eastern Europe had most of the confrontations with the Mongols).

The Mongolian control over China starting with Kublai marked the Yuan dynasty, a relatively short lived dynasty that was marked by famine, natural disasters, and overall unhappiness. Eventually, the Ming dynasty arose out of rebellion and drove the Yuan north, but they continued to be an issue until the early 1600s.

So, to answer the question, most of this time was spent rebuilding, reconquering, and fortifying, lest another major Steppe invasion occur. The setback to China was pretty enormous.

EDIT: I also forgot to include that China itself is not really known as a major sea-faring nation at this time. Going back to Kublai Khan, there was a major push to try and invade the Japanese by the Yuan. A large fleet was assembled multiple times after the conquering of Korea. In the first instance, much of the fleet was destroyed after sailing out to sea from Japan in an attempt to not be stuck on the beaches. The second attempt was marked by the famous Kamikaze. Again, catastrophic on military development and power projection