Why do most people in Chinatowns speak Cantonese instead of Mandarin?
The migrants from China to America took place largely in the 19th and 20th centuries. Most of them came from the southeastern part of China known as Canton (present-day Guangdong, just west of Hong Kong) ; hence, Cantonese was the common language oken. By mid-century, many Chinese migrants came from other parts of China and spoke a variety of dialects, but Mandarin was fast becoming the national language at that time.
