Here's why:
* Population: Mandarin is spoken by over 1 billion people, primarily in mainland China, Taiwan, and Singapore.
* Official Language: It is the official language of China, Taiwan, and Singapore.
* Regional Prevalence: While other languages like Japanese, Korean, and Vietnamese are also spoken in Eastern Asia, Mandarin's sheer number of speakers and its status as an official language in key countries make it the most widely spoken.
It's important to note that "Eastern Asia" is a broad region, and the language landscape varies significantly.