Why is Mandarin the largest Sinitic language group?

Mandarin is the largest Sinitic language group because it is the native language of two-thirds of the population of China. Historically, the Sinitic language area has continuously expanded from the "Middle Kingdom" around the eastern Huang He (Yellow River) to its current size, with Mandarin serving as the dominant form of speech across northern, central, and western China.123

Beyond its geographic prevalence, Mandarin—specifically the dialect of Beijing—forms the foundation of Modern Standard Chinese. Following the May Fourth Movement in 1919, which promoted the use of vernacular language over Classical Chinese, significant linguistic planning was undertaken to establish and spread this standard form. Consequently, millions of people whose native languages were other Sinitic varieties or non-Chinese languages learned to speak and understand this common language, further cementing its status as the most important language group within the Sino-Tibetan family.145