I don't understand how Seattle hasn't had a big housing problem compared to the valley or Vancouver. They have a decent tech sector and they're in close proximity to Vancouver.
Simply: It's more difficult to emigrate to the US as a foreigner in many key ways, coupled with the fact that it's a slightly larger city with a much stronger and more diversified economy.
(I am Chinese, is living in Vancouver, and know many mainland Chinese.)
In the eye of a mainland Chinese, an US citizenship is worth significantly less than Canadian citizenship. Take it for what you will, the bottom line is, no one wants to be an US citizen.
The difference in application fee is dramatic, as a Canadian citizenship application fee (to the middleman) costs 3x more than the US counterpart (Green Card).
I think it's coming. Rents have been climbing at a fast rate. My wife and I are paying close to $3K for a two bedroom + den in Bellevue, and this was the cheapest we could find. When we first moved in 3 years ago, our rent was just a little over $2K.
It's much less geographically constrained than Vancouver and it has fewer tech millionaires than the Valley. It has also had fairly significant price gains over the last few years though.