Welcome to the Clusterfuck. Since Seattle is technically under UCAS jurisdiction they are essentially an island. Surrounded on all sides by NAN members. Seattle looks at what happened to California and the JIS and is acutely aware of pacific interests in the city. Since it is so far way from UCAS it has been divvied up between megacorps perhaps more than any other sprawl. In short this makes Seattle a runners paradise. It is a place where all the corporate intrigues play out because it is essentially lawless.
Of course this lawlessness is a double edged sword. Organized Crime has moved in faster than than VITAS spreads. Gangs control many areas of the non corp non government sprawl. And Smaller PMC’s help make sure certain sections of the sprawl are well guarded for well to do due people that don’t want prying eyes.
Combine this with Humanis Polis, Mothers of Metahumans, Eco-groups and every other interest group you can think of looking to take advantage of the chaos and you have more competing interests then anywhere else. Add in the fact that Im sure NAN would love to add another sprawl to their territory and you have constantly shifting alliances and boundaries. This makes Seattle perhaps the place with the most opportunities for a runner and the most ways to get killed.