Why does Zhenjiang, Nanjing speak Jiangnan and Subei dialect?

Nanjing Zhenjiang speaks Subei dialect, which was caused by the Yongjia Rebellion in the late Western Jin Dynasty. Before Yongjia went south, Zhenjiang in Nanjing belonged to Wuyu District. There are more than 900,000 immigrants from Yongjia to the north, far exceeding Zhenjiang natives who speak Wu. Jianghuai Mandarin brought by immigrants from the north has been extended to the west of Maoshan. To the east of Maoshan is a rich land in the south of the Yangtze River, which is difficult for northern immigrants to enter. After several generations, the Wu dialect in Zhenjiang, Nanjing finally declined.