Why does Zhenjiang, south of the Yangtze River, speak Subei dialect and Nantong, north of the Yangtze River, speak Sunan dialect?

According to textual research, Wu dialect was spoken in Ningzhen area of Jiangnan before the Eastern Jin Dynasty. During the Yongjia Rebellion at the end of the Western Jin Dynasty, there were more than 900,000 northern immigrants in Du Nan, Guan Yi, far exceeding the indigenous people in Ningzhen area. The northern dialect brought by immigrants is obviously superior to Wu dialect. After several generations' development, Ningzhen area has been transformed from a pure Wu dialect area to the present northern mandarin area. On the other hand, Nantong in Jiangbei speaks Jianghuai Mandarin, but Tongdong in Haimen Qidong, Nantong is a Jiangnan immigrant in Wuyu area in history, so Tongdong in Haimen Qidong still speaks Sunan dialect until now.