Why could China keep most of the lands it conquered after 17th century but European powers could not?

The Europeans got no land but concessions from China, mostly in trade apart from signing over Hong Kong to the British which was about it. The europeans could not conduct a a land invasion into China proper, the defeats were mainly in naval engagements and even then the Qing managed to defeat the Portuguese, French and Dutch on several occasion so the idea that the European powers could gradually eat up China is ridiculous. Finally by the end of the Chinese civil war, the Communist had amassed an army of over 7 million, which had extensive experience in battling the massive Japanese invasion and winning the civil war, subjecting Tibet and Xinkiang under their control was a matter of will, the European situation was rather different, instead of strengthening they were greatly weakened and new powers rises to take their place, chiefly the United States, Russia and China.

In order to attack China, "European" powers had to send armies "halfway" around the world. That is a difficult way to "project" power even today, let alone in the 19th or early 20th centuries. It's fairly easy to send a "punitive" expedition, but much harder to conquer and control a country under those circumstances even with a large technological advantage. Think of the U.S. vs. Vietnam, and imagine how much easier it would be for the U.S. to conquer nearby Canada, it's technological equal.

Another thing was the relative populations. The Qing population of perhaps 300 million was about ten times that of most European states. Even with a technological advantage, it may be easy to "beat" but hard to "hold" a more populous country. Britain did it with India, but just barely. China is larger, further away, and more organized.

On the other hand, the indigenous populations of China's "colonies" in Tibet and Sinkiang were in the millions, so the Chinese to indigenous population was on order of 100 to 1. Couple that with the fact that China is "just across the border" from them, and can move settlers in behind the troops to pacify them.

In short, one can see why it was easier for China to hold onto nearby sparsely populated colonies than "Europe" with densely populated distant colonies.

The answer has to do with demographics based on agricultural productivity.

The sedentary 'Han' Chinese could spread into the lands of their conquerors and out-breed and out-produce them.

Thus, though the Mongols and Manchus and so on conquered 'Han' China, increasing proportions of their own homelands came to be ethnically Chinese.

In the Manchu case, there was already a process of elite assimilation even before the conquest and though the Manchu/Han distinction was instrumentalized for political purposes, the Japanese attempt to create a puppet 'Manchuko' was bound to fail because 90 percent of the population already identified as Han.

A separate point has to do with the difference between the Stalinist policy on Nationalities- whereby non Russian states were made bigger so as to include large Russian or other Russophile ethnic minorities- and the Chinese lip service to the same concept which masked massive, subsidized, re-settlement of Han Chinese and the categorization of vast classes of subject people as 'class enemies'.

I should point out that China is not ethnically h*m*geneous. It's just that 'Han' characteristics, no matter who adopts them, have superior demographic outcomes.

The annexations of Tibet & Sinkiang were by no means a foregone conclusion. Had the Soviets been suspicious of the regime in Peking, they could have kept Sinkiang as a buffer.

Similarly, suppose India had chosen to ally with the US, then it could have kept the Chinese out of Tibet by reason of superior air-power and the shelter provided by the U.S nuclear umbrella. India however supported China's claim to Tibet because it considered the U.S the greater threat.