Neither option is in any way, shape, or form ideal for a nation. But it seems like being completely conquered would be more humiliating then the terms the Japanese and Europeans forced on them. Was it do to the fact they were outsiders? Because they were white? Why?
"Century of Humiliation" was only coined around 1915 during a spike of a renewed national sentiment.
"Why" is a had question to answer, but in terms of National Identity, it is a relatively modern concept.
That said, the Mongols Yuan and Manchu Qing also used China as a seat of power, rather than dividing and plundering back to their home nations.
The Yuan and Qing were both assimilated into Chinese culture and ruled as "Chinese" legitimate Dynasties through the concept of "Mandate of Heaven" and further respected court traditions as offici al monarchs.
Kublai was Khan of the Mongols but also Emperor of Yuan China. He lived and slept and ate in Beijing. In a sense, the Yuan and Qing werent simply "part" of China, they were China.
In contrast, the European powers divided China and sought to enrich their home nations of goods from China while leaving "ruling" and leftovers to a crumbling Qing, not take the throne. They had no intention of presiding over China and its people.