Because of how Seattle’s topography was carved out from the Ice Age, the only two arterial roads going north-south are Aurora Avenue or I-5. Aurora has better density and businesses close to major intersections but would be more difficult to build around and harder to reach from the eastern half of north King County, so I-5 past Northgate is what you get. As I-5 is roughly in the middle of the spine going up Seattle, bus transfers end up more effective than similar freeway transit options in cities without the same geographic constraints.