Every few years, the local mass transit company (Metro) goes through funding problems. Why? Because much of the funding comes from 2 states, DC (the city, not the federal government), and all the cities and counties within the area covered by Metro. And every time it happens, everyone is fighting over how much funding should come from what source. (Riders also pay fares.)
Metro has been slowly but continually raising fares on riders because funding from the government sources is just not sufficient to cover operating costs. Also, if all the municipalities raised their taxes to cover Metro, the people who don't use Metro would be up in arms! It would not be a small tax increase. As I said, Metro has to charge riders more and more to cover the shortfalls from the governments. If all of a sudden, income from fares disappeared, the costs to the governments would be dramatic.
(And argument can be made that some of the counties could afford the tax increase, and that is true. But not all of them could. It would be a huge burden on some of the counties because they just do not have the tax base to cover it.)