It's actually quite interesting to consider regional preferences for cars and motorcycles. As a West Coast dude who has traveled all over this great land, I have noticed how different these preferences are. My brother lives in Wichita, KS and when visiting there you would think that Japan and Germany barely imported cars as everyone seems to drive an American made vehicle. Whereas on the West Coast the ratio is like 10-1 foreign to US in the cities.
With motorcycles I wonder how much of it is weather or local riding conditions. As speculation the major demarcation here in Seattle seems to be the Harley people and the non-Harley people, and the non-Harley people certainly dominate from what I can tell. Loads of guys ride BMW's of all types and the local mega-BMW-dealer seems to do a brisk business. There are lots of Honda's, Kawasaki's, Triumph's, and of course Yamaha. Lots of Ducati too. I don't know about other regional motorcycle preferences and how they relate to "Buy American" sentiments and/or machismo associations with Harley.
The demographic of guys and gals who ride the ferry with me includes a fair number of white collar professional sorts who choose to ride for all the ferry/money/time reasons I've gone into before here. It's a pretty affluent crowd, all things considered.
I'd also be interested in the per capita ridership numbers by city or region.
If you are ever in the area, you should really get on the ferry during commute hours going to Bainbridge from Seattle. Make a nice ride out of it. The boat is filled with every type and vintage of bike you can imagine. Really an impressive display of machinery all in one place.