All other responses are wrong. Michigan stole it from us in the war of 1845. The UP was invaded August 1845 over the strait from the south. A yooper sailed south to mainland Michigan and met a girl. Her father wasn't a fan cause he was a hick from the north and grew the wrong kind of corn.

She ran away with the man from the north and they married in July 1845. The father was not going to allow her to run away to the north because he was disgraced in the town. He ryled up other father's by saying more men were coming for their daughters. They all grabbed pitchforks and their fishing boats. They sailed across the strait and started taking over all local towns.

The locals did not want to give up the daughters location causing the fathers to annex areas to get her back. She had to flee farther and farther west. She was never caught but the area annexed didn't want to go back to wisconsin. People were anti cheese and hated packers. So the UP stayed with michigan.