White flight away from urban areas; combined with the fact that Republicans core base is well-off, married, white, religious families, pretty much explains most conservative suburbs.
In the Seattle area, the conservative areas are traditionally Auburn, Bellevue, and Mercer Island. Lots of well-to-do tech workers who try to avoid the taxes of Seattle itself. However the conservative hold has been slipping there, at least federally, as social issues become more of a deal-breaker.
You know, the more close-to-Seattle suburbs were once bellwethers for the State as a whole. Now they are solidly Democratic. These suburbs swung King County, which in turn swung Washington. Now the state is rock-solid Democratic on presidential level.