Why is Yunnan a northern language family?

Because Yunnan was a place dominated by ethnic minorities before the Ming Dynasty, a large number of Han immigrants entered Yunnan from the Ming Dynasty. Because immigrants come from different places, they communicate with each other in Mandarin. In the long historical development process, Yunnan Chinese dialects based on Putonghua have gradually formed, and Putonghua belongs to the northern language family, so Yunnan Chinese dialects belong to the northern language family accordingly.