As someone who lives in the lower peninsula, I can tell you that Wisconsin hasn't conquered the U.P. for the same reason no one conquers Russia. They can just retreat into the snow and let the wolves eat you.
I thought it’s because there is nothing there that anyone wants. I always thought Wisconsin was paying them to keep it. I can think of entire US states I’d be willing to pay France to take over.
It depends. If you're into hiking, fishing, hunting, and snow, it's a paradise. Lots of folks from the lower peninsula go there for vacations in the summer and fall.
Yooper here. Your example is on the money, but perhaps moreso than you know: The UP is also heavily populated by Finnish-Americans, which helps explain the cultural resilience and independence as well. The same dimensions that have kept Finland independent from Russia, keep the UP independent from others - and, holy wah, if I had a nickel every time we talked about breaking away from the trolls South of the bridge while at deer camp...