I'm not an economist but I do wonder if the reasons some of those places have higher mobility is because the previous generation in those areas have relatively low income jobs (i.e. mostly agriculture) and many of the current generation have a "gotta get out of here" mentality leading them to better paying opportunities elsewhere. I know I felt that way living in the mid-west in early high school.
I'd like to see a map of how many of these income reporters moved away from their locales and to where and also what professions they picked up.
The mobility those regions demonstrate is better than average for all income classes, and those income classes are defined nationally, not locally.
Of course, that "gotta get out of here" feeling isn't limited to lower income kids, so it could be a factor. Certainly it's typically good for your earnings if you're willing to move where you can make more money. But then why isn't upstate New York dark blue? (Sorry, upstate New York!)
I like your theory. I think the schools in those regions are generally pretty good, and due to agricultural consolidation and general urbanization, most kids in those areas see the writing on the wall.
I'd like to see a map of how many of these income reporters moved away from their locales and to where and also what professions they picked up.