China had a saying 胡无百年之运 that roughly means "Barbarians Cannot Rule for a Hundred Years", if you look at history at the states like Yan or the Yuan Dynasty this applies fairly well. So how did the Manchus rule for so long? They completely adopted the government and bureaucratic systems of the Ming Dynasty. By the 19th century the Manchu nobility could barely speak Manchu and were basically completely sinicized They were no longer the nomadic at the start of their conquests. The same is true for the Liao and Jin Dynastyies which also adopted Chinese customs, language, etc.

Empires like the Huns were just nomadic tribes that were united under a charismatic Ruler (i.e Atila), that didn't have the structure and logistics to sustain itself like the Qing Dyasnty.

The key to the Manchu's conquest of and hold onto power in China was because they won the support of the Confucian scholar-gentry class. The Manchus cast themselves as protectors of the Confucian tradition, more specifically, the Ming tradition, to the point that their government and institutions was basically a carbon-copy of the Ming's. Unlike the Mongol Yuan, the Manchu rulers also did not heavily discriminate against the native Han population and they worked to pacify a country that was ravaged for years by civil war. In the 1650s when Koxinga's Ming loyalist forces attacked Nanjing, they were surprised that the native population did not rise up in revolt against the Qing. During the Revolt of the Three Feudatories, the gentry class once again chose to support the Qing rather than Wu Sangui. This becomes especially important during the 19th century, when it was Confucian scholars and gentry leaders such as Zeng Guofan, Li Hongzhang, and Zuo Zongtang who led their own militia against the Taiping and Nien Rebels and saved the Qing from destruction. Contrast this with the late Yuan, when the Confucian elites who were instrumental in turning back the first ride of the Red Turban rebels cast their lots with Zhu Yuanzhang and Zhang Shicheng instead, leading to the collapse of Yuan authority in the south.