Funny how easy it is to pick up books locally in just about any state that have newspaper accounts of hairy man encounters going back to the time they were first settled. Find these all the time.
The PNW was, though, where bigfoot first captured the public's awareness, first in '59 with the widespread publication of track finds, then again in '67 with the Patterson video.
The PNW isn't the origin of bigfoot, but it is the origin of modern bigfoot coverage. So the phenomenon you postulate is a function of media coverage, rather than origin of the species itself.
People have been encountering them for centuries all around the country, and the Native Americans all over the country describe them in their history. Thing is, all of the history and reports were mostly local. Accounts simply didn't spread very far.
When the BFRO was founded, it provided a means for people everywhere to report their experiences, and so, suddenly, bigfoot were everywhere. Again, it wasn't because they abruptly appeared all around the country, it was simply that people all around the country finally had a means to file reports.
But to disprove the premise of the OP, all one needs to do is examine Native American reports of bigfoot around the country. This neatly eliminates the influence of modern media from the map.