Basically I think the message of this is stay the fuck out of the midwest.
I think It’s less that the supernatural activity is focused there, and more that Sam and Dean are most comfortable in the Midwest. It’s got the weather patterns they like best, the right amount of moisture and temperature for their sensibilities. (For the same reason, immigrants tended to move to states most like their homeland: Scandanavians to Wisconsin, Germans to Indiana and Ohio, British to New England, etc).
Meanwhile other hunters will tend to stay more in their own corners of the map. And the same is true, not only for the US, but for the entire world. Every hunter has their own domain, that place they like best, and they’re probably going to stick there, because the world is just too big to cover the entire thing.
It’s also noteworthy that a good deal of their work looking for cases is done via newspaper. The Indianapolis Star might mention weird things in Ohio or Illinois, maybe even Kentucky, but it’s not going to tell you about stuff going on in Arizona unless it’s REALLY WEIRD. The same goes for newspapers pretty much everywhere else, so most of their movement is going to be in relatively small jumps, rather than grand cross-country tours. And you’re just not going to find case-worthy stuff in national newspapers like the New York Times.