There appear to be a number of reasons. Among these are land development patterns. As our cities have expanded and the majority of us chose to move to the suburbs, public transportation has become less of an option. Simply, public transit does not serve where the majority of people live, work and play. Today, that is the suburbs.
Congestion is now greatest between suburbs, and no longer the suburb to city commute.
Moreover, our global 24-hour economy requires that people commute to and from work around the clock. Our current understanding of public transportation does not respond to these needs entirely. Transit authorities are making great strides. They are expanding services, using new technologies to improve scheduling. Anything this large will certainly affect a region - its urban form, environment, quality of life and traffic patterns.