Transportation is still a trade off between convenience and price. I think a lot of the people not living within biking distance are probably also living in an area where public transport is a serious inconvenience compared to other options.
I live in the Cleveland area, and even living in the downtown area, I could imagine it being a legitimate sacrifice to live without a car. While the downtown area is growing in residence and support services, such as grocery stores, are being built downtown, it's still tough. Also, while some businesses are moving their offices downtown, other companies, especially large ones like Eaton, have moved their substantial downtown presence to the suburbs, so the problem is happening in both the residential and commercial side of real-estate. I'd have to imagine many small and mid-size cities suffer from this.
I just don't see a solution that is going to get people to cluster more, given our culture. I think the long-term solution is electric cars and the possibilities that shared, highly convenient, self-driven cars present. So instead of changing behaviors drastically, we use technology to make the required changes much smaller.