The short answer as to why no-one has ever conquered China is size and population. But there is also the nature of the Chinese people. It has been possible for leaders to enthuse its population with aims that, although sometimes harsh, have wrought immense change.
For instance, the moral duty to kill any fly seen led eventually to the removal of this irritant, and the one-child policy, to a situation where over population was a threat overcome. With this ability to mobilise mass obedience, China has been able to move 'as one' and not weakened by internal divisions seen in European countries.
China is a country that has always been powerful and has a very long history. Since it has been established for a long time, it dates back to the ancient times. Today, China is the largest country in the world based on its population. In regard to this statistic, this makes a country powerful and hard to conquer. Usually, if a country has a strong government and large population, then their military will be large and strong as well. One way that China has been able to withstand and cause attackers to retreat is by stopping the attackers.
Sometimes, China was forced to go to war, but other times they would find a way to protect themselves. The Great Wall of China was built in order to stop the Mongols from coming into the country.