Why are some areas not far apart but the dialect differences are too big to understand, while others are similar?

In fact, there are many immigration problems. Here, across a river, our accents are actually different. In history, there may be many immigrants in an area, such as Huguang filling Sichuan and Shandong going to the customs. Where they live together, their accents will be complicated, which will leave a legacy. There is no need for any city to have a dialect. For example, there are immigrants from Jinan, Qufu, Qingdao and Yantai in our city, and their accents are naturally different.