Free public transport is a can of worms; in general, free tends to attract walkers and bikers but not drivers, since price is not really how a car competes with public transport. And attracting walkers and bikers would probably be a negative. There's also a negative cycle where some walkers and bikers will switch because the free service is good enough, so the services become overcrowded, so more service is provided and the increased frequencies make public transit even more attractive to walkers and bikers. So you end up increasing costs quite a bit with no increase in "revenue".

There's also the fact that in even moderately busy systems, the amount of fares collected is not insignificant. As an example, New York's MTA: http://interactive.nydailynews.com/project/mta-funding/

$6B out of $15B is quite a lot of money. Raising taxes to cover $6B in lost revenue would be quite the feat. And why wouldn't that $6B be better put towards housing, or hospitals, or schools, or even better public transport services? It's generally better to make services more useful, than to just make existing subpar service free.