Why is Sichuan dialect a northern dialect?

I am a Sichuanese studying vocal music. Our major pays special attention to enunciation. Undergraduate students should learn singing language expression (similar to the broadcast host textbook), Italian, German and French pronunciation, so almost everyone who studies vocal music is very sensitive to language. Personally, Sichuan dialect is not a local dialect, but a mixture of many dialects, with obvious traces of north-south integration. The overall pronunciation of Sichuan dialect (consonants, vowels, without intonation) is basically the same as that of Mandarin, which is also an important basis for the division of northern language families. Generally speaking, most northerners can understand. Among them, there is no tongue coating in most areas of northern Sichuan and eastern Sichuan, and there is tongue coating in southern Sichuan and western Sichuan (there is no tongue coating in Chengdu and there is tongue coating in surrounding areas such as Pengzhou). Then, the pronunciations close to the southern Cantonese family are: Yi (Yi) Urine, Yong (Yun) Yuan, Jie (Gai) Hand, Xie (Hai) Zi, Yi (Gan) Fu, Furong (Yong), Jie (Gai) Road, Sleeping (Gao) and so on, all of which are in the ancient town of Luodai, Chengdu. In addition, Sichuan Opera originated from Kunqu Opera, which proves that Sichuan is a multi-regional, multi-ethnic and multi-cultural province. Spicy and delicious hot pot and warm and elegant boiled cabbage at the state banquet reflect the enthusiasm and tolerance of the Sichuan people. Local dialects are a kind of culture, a kind of inheritance, bearing the attitude of generations to life, hard-working, kind and warm affection.