Two years ago the GTOAA Nationals was held in Bellevue, a suburb of Seattle. If memory serves it was only the second time it had ever been held west of the Mississippi.
The local GTO club worked their collective butts off to find a venue, organize the main event, setup side events (including a post-show Alaskan cruise, and (most importantly) to get as many local Pontiac owners there as possible.
They even came to Jet City Fieros meetings twice to invite us. It was a steep show for a Fiero owner- $100 for the first car. We started with almost no interest, and ended up putting 13 cars in the show. This would not have happened if they hadn't been so inclusive and worked so hard at it.
My understanding is that it was the best attended GTOAA Nationals show in many years- over 500 cars. The concours section was mind-blowing. One attendee who drove from the east coast stopped at a Pontiac museum and picked up an old Pontiac inflatable balloon that was displayed There was also a ton of vintage signage.
Putting on a show like this is a huge amount of work. Much of which, if things go well, is never noticed. No matter how great and full of possibility a venue/location is for a national event, it will suck if the locals aren't enthusiastically signed up to put it on.
Seattle/Portland and NorCal would be fun locations with lots to do- great roads, tracks available for autocross, go-karts, amazing scenery, etc. But absent a well-organized, energetic group to scheme, organize, evangelize, and breathe life into the event the potential and need to have it at a specific location or region isn't relevant.