Why is the dialect in Guilin the same as the urbanized dialect in Guiyang County, Hunan Province?

Southwest Mandarin, also known as Shangjiang Mandarin and Huguang Mandarin, is a dialect of Chinese Mandarin. It is the main Chinese dialect in southwest China, Sichuan, Chongqing, Yunnan, Guizhou and most neighboring Hubei, southwest Hunan, Guangxi Guiliu, southwest Shaanxi, Xinyang, Henan and northern Southeast Asia (including Kokang, Myanmar). Some China people in Laos and Viet Nam also use it. According to the definition given in Atlas of Chinese Language, Southwest Mandarin is a Chinese dialect with a certain tone or four tones similar to Chengdu, Wuhan, Chongqing, Guiyang, Kunming and Guilin.