Looking at a map, it seems like grouping it with Wisconsin makes much more geographical sense than grouping it with mainland Michigan.
That question was actually answered a few days ago here by u/SharksWithFlareGuns : https://www.reddit.com/r/AskHistorians/comments/jo8iem/michigans_upper_peninsula_is_connected_to_the/gb78q9z?utm_medium=android_app&utm_source=share&context=3