Even though Portland is the "number one transit city" in the country, Seattle has a higher rate of commuters using transit.
Sightline credits the difference largely to land-use issues:
Largely through accidents of geography and history, Seattle has a handful of highly concentrated job centers, particularly downtown and the University of Washington; and transit is a particularly effective commuting option in neighborhoods with high employment densities. And at the same time, high parking costs in Seattle’s downtown discourage drive-alone commuting.
I'd add another land-use issue: Two of Portland's biggest employers—Nike and Intel—are out in the 'burbs. That probably ups our driving rate compared to Seattle's.