Absolutely right about Jackson County which is very close to Ann Arbor but is firmly within MSU’s orbit.
What I mean by “geographic factor” goes to two things: one, where does the University of Michigan invest most heavily in student recruitment, and two, how is that perceived in the rest of the state. The university is not a passive player here. It actively recruits, especially in the high-end high schools in the more affluent pockets of the Detroit metro region and Washtenaw County. And it draws its student body very strongly from those places, especially from the better schools in the more affluent pockets of Oakland and Washtenaw Counties. And I am asserting that this is perceived by at least some people in other parts of the state as an indication that the university is less interested in people like them—both less interested in students from less affluent backgrounds, and less interested in students, even highly qualified students, from outside the Detroit metro area. And so some of those students, even top students, gravitate more toward Michigan State, which is perceived as both a less metro Detroit-centric institution and a less high-SES-skewed institution.
I actually think this also plays out in athletic recruiting. Michigan is competitive with MSU for top athletic recruits coming out of the Detroit metro area, but Tom Izzo pretty much owns places like Flint, Saginaw, and Lansing.