1) It is a relatively new city, built on money generated by proximity to Microsoft, and because it is new and expensive and populated almost entirely by a college-educated population, it has not (yet) accumulated an underclass.
2) Seattle/Bellevue/Redmond started out as a heavily Scandinavian logging and coal region, but never faced the migration of black communities from farms during automation agriculture and wartime. Consequently, it is a very “white” area with Asian and Hindu minorities.
3) Neither Seattle is one of the traditional Gateway cities for processing first generation or two immigrants.
4) Also, the Northwest is “hard to get to.” The weather is gray and humid. It’s excruciatingly dark most of the year. And so it attracts a certain class of people.
There’s a joke about Bellevue being the “Green Zone”: a great place to live among the rapidly declining majority of the country simply because it’s a rare man-made economy, and it’s too expensive to survive and there’s no demand for unskilled labor (those people live up north in Bellingham or down south in Renton.)
As impolite as it is, if you want a good region, the main problem is keeping the lower classes out. Because the lower classes exhibit undesirable behavior that diminishes the quality of territory, institutions, capital, norms, schools, stores, neighborhoods, homes, private property, and family life.
Europe is a huge open-air museum because for thousands of years they have practiced merit-based land distribution, manorialism, late marriage and childbirth, a ban on cousin marriages, and have taken about 1% of the total population every year. As a result of the Industrial Revolution, much of Europe descended from members of what we would call the middle social class.