Why do you think it's harder to track mass transportation? Things like clipper cards, credit cards, etc will be able to log the data. If they also offered wifi it will help as well.

Don't know why they didn't do mass transportation, but my guess is legislation and unions. Getting rid of personal cars and human driving taxis is private industry. Mass transportation will have a large percentage be public sector workers and union workers.

If this is the path of least resistence and is proven to work, they will most likely go after mass transportation as well.