It's all about heat, salt air, and and UV damage...
NW boats have far far (far!) less of these than the cooked-well-done FL or SoCal boats.
Rigging lasts longer and hulls look better at 20 and even 30 years of age.
On the minus side, if NW owners fail to do their due diligence in rebedding deck fittings and toe rails, the boats can suffer major deck core rot........
(I once helped move (in protected waters!) a 70's Cat 30 with a foredeck that bounced like a trampolene.)
There are likely lots of other reasons and annecdotal evidence, but this is just a start.
And as others have said, there's always supply-and-demand.
Happy shopping!
As for us, when we bought our present boat, we split the difference, distance wise, and had one trucked back up here from SF.
One more random thought... when you shop for a boat model over 25 years old, be aware that the price will vary by 200% or 400% from the un-maintained "project boats" to the (Rare!) turn key ones. I personally know of a fully-maintained Cat 30 that's for sale in my area now for 32K. While you can find Gawd-awful ones for under 7K, you really do get "what you pay for" or.... less....
This one,
http://portland.craigslist.org/mlt/boa/3607495842.html
is
only for sale because the owners have decided to move to a trawler. And no, I am not a broker and have no personal interest.