The problem in North America is sprawl and shopping centers. Our cities, for the most part, have grown and are growing outward, not upward like a lot of European cities. Density in the U.S. cities stays about the same and just sort of spreads out instead of density increasing. It seems each community has its own main shopping area so there is little reason in many cities to go to the actual city center unless you work there or are going to a sporting event or museum.
The only cities I can think of that are exceptions and have good public transportation systems are San Francisco, Chicago, and New York. Notice that they are all large cities with large city centers. I'm not sure if LA or Boston (also very large cities) have good public transportation systems or not.