Why do people in eastern Fujian and southern Zhejiang also speak Minnan?

From the Qin and Han Dynasties to the end of the Southern Song Dynasty, the Han people entered Fujian from the Central Plains on a large scale for reasons such as fighting and taking refuge, bringing the culture of the Central Plains, including the customs and Chinese at that time, and assimilating the people of Fujian and Vietnam. Chinese and ancient Fujian-Vietnamese blended to form Minnan language, and Zhangzhou and Quanzhou are the birthplaces of various Minnan languages referred to in linguistics. Minnan dialect is located in southern Fujian, Taiwan Province Province, Chaoshan area in eastern Guangdong, Hainan Island, Longyan City and Zhangping area in western Fujian, Fuding area in Ningde City in eastern Fujian, Cangnan, Pingyang and Dongtou areas in southern Zhejiang, and some countries and regions in Southeast Asia. During the late Ming and early Qing Dynasties, a large number of Minnan people (mainly Longxi, Haicheng, Zhangpu, Anxi, Hui 'an and other places) migrated to Cangnan, Pingyang, Yuhuan, Dongtou in southern Zhejiang and Fuding and Xiapu in northeastern Fujian.