Simply put, it is because the productivity level in the late Qing Dynasty has fallen far behind the average level in the world (especially in Western Europe).

Specifically, there are many reasons. Personally, I think the main reason is centralization. Different from the feudal system in Western Europe and Japan, the centralization in China is unique and highly centralized. It has completely controlled China society in politics, economy and culture, and made China economy based on small-scale peasant economy become an endogenous stable structure. If we discuss the deeper reasons, many reasons will come into view, among which the "agricultural war" system established by the Qin Dynasty and the threat of nomadic people in the north are more important.

In fact, when it comes to "decline", all dynasties are faced with decline, land annexation and nomadic invasion, as well as the rigidity of bureaucratic system, which is a stubborn disease in agricultural society. Of course, closing the door is the rule, but closing the door is determined by the endogenous pursuit of stability in agricultural society. In fact, the closed-door policy in Ming and Qing Dynasties has not been opposed by the domestic mainstream views, which can be seen.

There are many other reasons, such as China's Confucian governance, China's failure to establish an axiom system academically, China's emphasis on agriculture and suppression of business, China's property distribution system and inheritance system, and so on. But if I have to answer, I think it is centralized. Needham, a foreign scholar, has seriously thought about this issue and put forward the "Needham Mystery" in his History of Science and Technology in China. China Zheng Shimeng's exposition in The Influence of Centralization on the Development of Modern Science and Technology in China, I think, illustrates some problems.