Why does the Southwestern Tai group contain most Tai language speakers?

The Southwestern Tai group contains the majority of Tai language speakers because it represents a significant and geographically widespread expansion that occurred in comparatively recent historical periods.1

During the 1st millennium CE, the political strengths of the müang system—a social organization based on groups of villages led by a hereditary chief—enabled Tai-speaking peoples to move out of their original homeland. By the 8th century, they had expanded across much of northern mainland Southeast Asia and southeastern China. This migration continued as they filtered into the area of modern-day Thailand by the 11th century, eventually forming petty principalities.23

Today, the Southwestern group is the most widespread of the three Tai linguistic divisions, encompassing languages such as Thai, Lao, and Shan, and it accounts for approximately two-thirds of the total Tai-speaking population.41