Corruption.

They were very corrupt and the Chinese are a pretty degenerate bunch at the best of times.  They are gambling addicts and heavily into drugs, pleasure and money.  They cared little for the social welfare of those around them because of their pagan culture all based on superstitious nonsense.

Just look at the way they behave today.

The British found them easy to trade with because they are so corrupt and predictable.

Don't forget some European nations were barbarians before and at the beginning of the Middle Ages, including Germans.
They were also corrupt.

But what's caused the corruption??
Don't forget that China was also very advanced in some areas before European civilisations reached the peak.
What caused China to decline and decline to reach that level what you called "corrupt" ??

You're going back WAAAAAAAAAAY before the Qing dynasty there. By the time they came to power Europe was far ahead, and by the time they fell Europe was so far ahead it was running victory laps. In the 1800s the British invaded China just because the Chinese banned their opium, and destroyed them with very small forces. During the Boxer Rebellions due to persecution of Christians, the Great Powers invaded China and roflstomped it to put it simple. China had been declining for hundreds of years before the Qing dynasty came to power.

The causes of its corruption? Size of the nation, gigantic bureaucracy(and a culture and philosophy that basically celebrated bureaucracy), very low literacy rates compared to Europe, much less opportunities for enterprise than Europe, an unwillingness to adapt, many many reasons spanning centuries.

In James Walsh's book, "The Thirteenth, the greatest of centuries" he shows how this was one of the only times in modern history (Not including the Old Testament) when governments existed to truly benefit the people, because the State was an arm of the True Faith, and by and large, almost all citizens practiced the Catholicism to a large degree.  This marriage between Church/State is wonderful, when the Church that is practiced is Christ's; when society is based on and run as if everyone is truly responsible to God for all their actions; when charity of neighbor and the salvation of souls is the highest goal.

All other forms of government will be corrupted, eventually.  You ask why the Ch'ing Dynasty fell?  Same reasons as why Rome fell, why Alexander the Great's empire fell, why the British Empire fell - immorality, greed and lust for power.  No nation, empire or civilization can survive these things, which are all part of our fallen human nature.  Catholicism is the ONLY way a nation or empire can survive long term - because anything great is built through hard work, sacrifice and love of a goal - these 3 attributes CANNOT exist in an immoral people.  

Agreed, I'll have to have a read of that book at some point. Natural Law is not just a list of rules that we follow because the Church says so, although of course that's a very good reason to do something, but they're also the guidelines for how the human creature and the nation they live within can be as happy and contented as possible. Modernists hate that God bans things like fornication, as they say they are constraining rules which prevent them from having fun. But as we know, the number of lifetime sɛҳuąƖ partners is correlated almost perfectly with divorce and marital unhappiness. And marriage provides the most secure and best way to raise children, and as we see in the modern world of fornicators who don't marry we're left with children damaged and never fully matured due to fatherlessness. So it's clear fornication is not a mortal sin because God is "anti-fun", it's a mortal sin because it is both wrong and bad for the long-term happiness of everyone involved. It's like with food. If you decide to go pig out on McDonalds you'll be happy in the short-term, but long-term it'll ruin your health and therefore your happiness.

So, to sum it up, the best way for a person to be happy, and the best way for a country to have happy citizens( -> stability and strength for the nation), is to follow Natural Law. Abandoning natural law in favour of secularism or pagan religions is, well the hint is in the name, unnatural and therefore is the antithesis to a country's longevity and well-being just as an unhealthy lifestyle will make a person weak and die younger. It's no wonder we're seeing the Western nations fall apart all around us.