Why do so many Guangxi people speak vernacular?

Many people in the south and east of Guangxi speak the vernacular. The three coastal cities in Guangxi historically belonged to Guangdong until the 1960s. Therefore, Fangchenggang, Qinzhou, and Beihai all speak the vernacular. In addition, there are more people in Nanning, the capital. The vernacular is spoken, but the pronunciation and intonation of the vernacular differs between cities. The Guidong area, especially Wuzhou, has a close relationship with Guangdong, so the vernacular there is closest to the Cantonese vernacular. However, in recent years, due to economic development, foreigners have come to Guangxi and Guangxi to speak the vernacular. The number of people is decreasing, especially in the three coastal cities and Nanning. Among them, Qinzhou and Wuzhou have the highest penetration rate of vernacular.