In 1911, Outer Mongolia declared independence from China (Qing Dynasty), while Inner Mongolia never did. Why was this?

Hello! I'm currently writing my dissertation on Chinese nationalism so I can actually help answer this!!

Firstly, In Henry Kissinger's book "World Order", it is explained that:

"In one of history's most amazing feats of cultural imperialism; two peoples that conquered China, the Mongols, in the thirteenth century and the Manchu in the seventeenth, were induced to adopt core elements of Chinese culture to facilitate the administration of a people so numerous and so objurgate in its assumption of cultural superiority the conquers were significantly assimilated by the defeated Chinese society to a point where substantial parts of their home territory came to be traditionally treated as Chinese."

To relate to your question of why Inner Mongolia never declared independence, Kissinger says that when Mongolia conquered China, they had to assimilate in order to rule China. They had assimilated so much, that much of the traditional Mongolian lands just came to be seen as traditional Chinese lands.

Secondly, Kissinger's other book "On China", it is explained that:

"Mao set out to re-occupy the secessionist regions, such as Xinjiang, Inner Mongolia, and eventually Tibet"

Inner Mongolia did kind-of secede but that was due the Qing collapse, which led to Warlord-ism in China around in around 1916-28. Mao recaptured the region.

Why didn't Inner Mongolia declare independence too?

David Bello in "Across Forest, Steppe, and Mountain", it is explained that Inner Mongolia is closer to the Han core, and thus was the subject of a greater intensity of assimilation than regions further away like Outer Mongolia, Tibet and Xinjiang. Assimilation into Han Chinese is seen as "the Sinification of previously barbarian regions".

Fravel in "Strong Borders, Secure Nation: Cooperation and Conflict in China" continues this idea, explaining that from Qin to Qing, the Han core has been gradually expanding through the sinicization of non-Han groups resigning around the core, Inner Mongolia is closer and thus subject to greater sinizication than Outer Mongolia. We can even see this today! Western regions in China are more prone to conflict and general strife because it's more racially diverse, as in, the Han Chinese are not the crushing majority there (yet)!

Did the population of Chinese outnumber the Mongolians there, the way it does today?

Yes! By conquering China, the Mongols were suddenly a minority ruling a crushingly Han-dominated China! Rong Ma's "Migrant and Ethnic Integration in Rural Chifeng, Inner Mongolia Autonomous Region, China" tells of how the Han Chinese had been in Inner Mongolia during the Warring states period, but not in any significant numbers. When Outer Manchuria was seized by the Russians, the Qing saw the expansion of the Han Core as a necessity, encouraging Han farmers and settlers to migrate to Inner M. so it was not seized outright by a foreign power.

I'm open to answering any questions!!!