Most of them don't speak Beijing Mandarin. There's a slew of languages spoken in China, and Mandarin is the native language to only a small part of it.
Somewhat like if every state in America spoke a different language and official business was conducted in Marylandian.
Mandarin is more widely spoken that can be imagined. Check out this link. Scroll down to the chart.
http://www.nationsonline.org/oneworld/most_spoken_languages.htm
Not really.
Mandarin is a made up language. They took a dialect from around Beijing and literally made up a new language from it.
"Mandarin" was formed by scholars, not natural evolution.
It was compelled upon the population during the many revolutions of the late 19th through 20th centuries.
The invention of mass communication (telephone, tv, radio, etc) along with a nationalist movement made it so.
Dr Sun envisioned it. Chiang started it calling his national language "Guo Yu"...Mao continued it being different though calling it "pu tong hua"... (however mao distorted things badly-this is the basis of his cultural revolution). None the less people ended up learning mandarin on a wide scale basis.
It is not a native driven language such as the true dialects that have evolved over time.
I can explain the history of Chinese linguistic development and the development of their writing system(s) if you would like.