John Darwin gives it a shot:
The best answer we have is that it [Kiangnan and China] could not surmount the classic constraints of pre-industrial growth. By the late eighteenth century it faced steeply rising costs for food, fuel and raw materials. Increasing population and expanding output competed for the produce of a more or less fixed land area. The demand for food throttled the increase in raw cotton production. Raw cotton prices probably doubled in the Yangtze delta between 1750 and 1800. The demand for fuel (in the form of wood) brought deforestation and a degraded environment. The escape route from this trap existed in theory. Kiangman should have drawn its supplies from further away. It should have cut the costs of production by mechanization, enlarging its market and thus its source of supply. It should have turned to coal to meet the need for fuel. In practice there was little chance for change along such lines. It faced competition from many inland centres where food and raw materials were cheaper, and which could also exploit China’s well-developed system of waterway transport. The very perfection of China’s commercial economy allowed new producers to enter the market with comparative ease at the same technological level. Under these conditions, mechanization — even if technologically practical — might have been stymied at birth. And, though China had coal, it was far from Kiangnan and could not be transported there cheaply. Thus, for China as a whole, both the incentive and the means to take the industrial "high road" were meagre or absent.
In other words, who really knows? The excerpt is from Darwin’s new book After Tamerlane: The Global History of Empire Since 1405, which should be read by anyone…who…reads books with titles like that. It is most interesting on the Indian and Arabic collapse of the 18th century and on fitting the Russian conquest of Central Asia into the more general history of European imperialism. I didn’t find any revelations in the book, but it was consistently interesting and readable throughout.