If you're referring to the Manchu language spoken by the rulers of the Qing dynasty, then:
The Qing elite never assimilated into Han Chinese culture. Laws forbade Manchus from marrying Han and regulated business with them. The Manchu conquerors viewed themselves as fundamentally superior to Han Chinese because of their militaristic identity. The goal was to prevent the Manchu ruling class from losing this identity and becoming complacent. In many cities, there was essentially a Manchu enclave where Qing administrators and officials would live apart from the Han population.
The Qing court issued major edicts in five languages (Manchu, Chinese, Mongolian, Tibetan, Chagatai (Uyghur). This reduced the need for the conquered population to understand the Manchu language.
The Qing retained the metropolitan exam system used by previous dynasties, which was administered in Chinese and based on the Confucian classics. Thus, the state bureaucracy was like to speak a Chinese language, and it was simply more practical to conduct court business in the Beijing dialect than in the Manchu language.
Because Han Chinese nationalism was a driving force in the 1911 Revolution, the revolution was accompanied by massacres of Manchus and an almost genocidal drive to remove Manchu influence. Many Manchus who had served the Qing but lived in Han-majority areas were displaced. Thus, the Manchu language, which never gained a foothold in southern and central China was more or less totally displaced in 1911.
After the Communists came to power in 1949, they devoted considerable attention to the role of language in nation-building. The Beijing dialect was selected as the model for the national standard (what we call Mandarin), the Pinyin system of phonetic pronunciation was developed to teach this dialect to other Chinese speakers (since the Chinese script is logographic and contains no phonetic markers to assist in teaching a particular dialect or spoken language), and compulsory schooling was introduced. While the 1954 constitution affirmed the cultural rights fo ethnic minorities, the Beijing government essentially set the goal of teaching every single citizen to speak Mandarin. Initially, it was taught as a second language in schools, but today, it's pretty much the first language of instruction in most schools (if not in elementary schools, then definitely by high school).
Main source: Immanuel Hsu's Modern China, sixth edition.