I claim no definitive knowledge but here are my observations.
The staple flour-based filler of Northern China is the 'bao' steamed/fried flat bun that is partially folded.
It is eaten with everything with a sauce, stew or braised; especially in Winter. Some flour-based noodles also appear in northern cuisine.
Southern China go for rice and rice-based noodles/vermicelli as well as rice congee. However, somewhere along the time line, the refined Cantonese (southern) cuisine made it back North to be Imperial cuisine. China had moved its capital from north to south and back north.
Chang'an (Xian) to Nanjing to Beijing.........with interludes in Luoyang, Kaifeng, Hangzhou, Anyang and Zhengzhou.
So, the fancy thin pancakes started being used for wrapping the goose skin (later Peking Duck) as a delicacy with plum sauce and spring onions.
If you go to Beijing, they laugh at the skinny pancakes and still use the mantou (barbarian's head) bun. Of course, as a sop to 'refinement' the mantou these days are pretty flat (thin) in the fancy duck restaurants.