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.