Interesting! Without having a particular factual basis to go on, my immediate reaction is twofold-
1. On one hand, I think it would have to have some impact. People write about what they know, and the change of focus from the Midwest to the Pacific Northwest had to have an impact.
2. On the other hand, I also think that regionalism isn't nearly as important as it used to be in the United States. It's still there- but for various reasons, while regionalism persists, it isn't as much of a factor as it was in the 70s.
That's a verbose way of saying, "I don't know."