Reddit mentions of Dictionary of Alaskan English

I don't have my copy handy, and the libraries are closed for the night, so I'm going from memory on this. Refer to The Dictionary of Alaskan English.

In short, the term dates from before 1959, when there literally were 48 states. Hawaii was a territory, so it didn't factor into things. You also see "the States" mentioned in a similar context.

Incidentally, "going Outside" (capitalized O) is another common term, but it can be used for any Outside Alaska location, not just the United States.