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Every city on the west coast is a transplant city. It's something people in Phoenix say that is also true about Los Angeles, San Diego, San Francisco, Seattle, and every damn metro area on the west coast.
I watched the bandwagon grow for the Seahawks before my eyes when I lived up there and you didn't see a ton of opposing fans at their stadium.
I always just took it at face value that AZ was all transplants. So I looked it up and found this from the 2019 census. Bare in mind a good percentage of these may have been born elsewhere and lived in their state for 50 years.
45% of Arizona's population was born in another state. (I left out places like Delaware and Wyoming where the population would fit in my garden)
Nevada - 51%
New Hampshire - 51%
Washington DC - 48.6%
Colorado - 46.5%
Florida - 39.22% (Actually more than natives but a lot of immigrants)
Oregon - 43%
Idaho - 47%
Washington - 37%
Maryland - 36%
Virginia - 36%
South Caroline - 38%
North Carolina - 35%
Tennessee - 35%
New Jersey - 22% (Nobody want's to live their? Weird)
Kansas - 32%
Texas - 21%
Most of the rest are about 25-35% but I left them off because they are small or don't have a team. So It's not as if AZ are wildly different than anywhere else.
I don't see loads of opposing fans at Raiders or Broncos games.