-Cities are "greener" than suburbs
-New suburbs are easier to build than new cities. So, when "flight" occurs, it tends to flow either back into cities, or out to new suburbs.
-Traffic (more traffic makes living closer even more appealing)
-Proximity to California (as California gets more expensive, and Californians look for a different pace/better access to nature, they look up and down the coast. North seems the most appealing direction, and Seattle is larger city than Portland).
I think the only real fly-in-the-soup for cities is a concern for personal safety, borne either of fears of terrorism, pandemic, etc. Those issues are so large and hard to deal with, that they are pretty easy to get over.
The cities have it! And, Seattle has it in spades.