Why is Minnesota the only mainland state with an abundance of wolves?

From the Star Tribune in Minneapolis/St. Paul, Minnesota:

Long after wild wolves were wiped out of every other state in the “Lower 48,” dozens of wolf packs were somehow able to hang on and survive in remote areas of northern Minnesota.

While Sharon Carlson of Andover has never heard Minnesota wolves howling or seen them in the wild, it’s always impressed her that the state became the last refuge in the contiguous United States for the revered and reviled predators. But she’s never understood how Minnesota came to be so important for the animals.

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