Chicago, of course, has a few handicaps. One is distance. Chicago (like many eastern and midwestern cities) has suffered from a sprawl-and-abandon development pattern, and while Chicago has managed to revitalize its urban core (like New York, unlike Cleveland or Detroit), it still has a band of severe economic depression in the outskirts of the city and the inner suburbs. Seattle is hemmed in by water and mountains that limit the pure extent of sprawl, and its history and geography has been shaped less by violence and racism. To be sure, Seattle's geography as it is has been shaped by these factors, not to mention corruption and gentrification, but not to the extreme degree Chicago's has. So Seattle's "boom-burbs" are closer than Chicago's are — closer to eachother and closer to the urban core. I think Chicago's other big handicap is the success and popularity of its commuter rail system. Commuter rail is cheap to expand because it runs on existing freight tracks, and popular because it allows people to avoid stressful downtown driving and expensive downtown parking. It's also inflexible, incapable of serving "boom-burbs" that mostly are nowhere near train stations. Seattle's advantages here are twofold. First, almost all of its transit service uses buses, which are very flexible. Second, the same agencies are responsible for suburban commuter services and local services, so they have a greater incentive to cooperate than compete for funds. Agency splits and politics also probably hurt transit service in the urban core of Seattle.
2. While talking bike commuting, a friend said that cyclists with serious-looking racing gear seemed to make up a really high proportion of Seattle cyclists. That sort of serves to reason, I guess. Seattle has a lot of weather that would scare off casual riders, but very little that will deter a committed cyclist. Combine that with the US cycling industry's general disregard for serious adult cyclists that aren't racers, and you have a lot of people riding around town in racing gear, even though many of them will never race on a bike in their lives. Some sport clothing is even pretty practical for longer commutes (I wear bike shorts and a bright yellow pullover, which work great our misty rain), though I'll never understand why people buy gear with "sponsor" logos all over it.