So, why do you think there’s a difference between the Vale and the Northern clans? Is the ethnic division supposed to be doing all the work? The difference in geography?

Ethnic and religious issues, primarily. The Vale clans are First Men, the men who took their land are Andals; the Vale clans follow the Old Gods, the Andals follow the Seven. So there’s a really sharp cultural division that has hardened over thousands of years, and a real sense of alienation. 

Compare that to the hill clans of the North. Rather than a subjugated minority, they were brought into the realm by the Starks defeating the Flint Kings of Breakstone Hill, the same as the Umbers, Glovers, Ryders, Slates, Lockes, Reeds, etc. Rather than being isolated, they’re part of a pattern.

Add to that that the Starks have worked to integrate them – the hill clans trade their cattle and shelter for the winter in the shadow of Winterfell, the Lords of Winterfell come in person to ask for their support in war, and they’ve intermarried with the hill clans. Ned Stark’s kids have Norrey blood in their veins, and their great-grandmother was a Flint.