1. Geography: The metro area occupies a very narrow strip of land between the Cascade mountains and the Puget Sound, meaning that there is only so many places to feasibly build.
2. Most of the employment in the Seattle area is centered around fields that require an educated labor force. As a result, you wouldn't have large concentrations of "subprime" borrowers as you would in Orlando, the Inland Empire, etc.
3. For some strange reason, Seattle and Portland seem to attract folks who move there without jobs because they "love the environment" (ie nature plus other smug, liberal, well educated white folks). This adds to the demand side.
Seattle actually seems rather underpriced given the relative desirability of the area.