There is no simple answer to this question, but I've always wondered if it wasn't in fact China's strength and success as THE major imperial power in the region and as the center of a complex system of trade and diplomacy that made its response to Western aggression less successful in the long run.

Japanese leaders in the late 16th-early 17th centuries had been spooked by the viral spread of Christianity and by the potential for European trade and proselytization to open up networks of cultural and political control that Japanese warlords had previously dominated. They then famously limited their borders, shut down overseas Japanese travel, and outlawed Christianity completely as part of a broader set of policies aimed at pacifying Japan and locking people into place. This was a move made out of relative international weakness and a determination on the part of the Tokugawa to avoid further civil wars after a century of conflict.

The Chinese under the Qing dynasty on the other hand were a strong, multicultural empire that dominated the East Asian system and could dictate the terms of their interactions with all outsiders, at least up until the Opium Wars. The coherence of the political and social ideology that undergirded China's world view combined with the many centuries of successful dominance in the region would have made it virtually impossible for anyone in Beijing to anticipate the extreme determination and methodology that the European powers would use to pry open the doors to Chinese markets and resources. So, perhaps it was the strength and historical success of the Chinese system--even through multiple regime changes/dynastic shifts--that made their response to 19th-century global shifts seem, in retrospect, disastrous.