One theory is if its free, more people might hop on the bus and use it for a block or two since they know its free and they don’t have to keep up a current transit pass. This basically catches the car users who might occasionally use transit but aren’t familiar with the pass system as it is. Free fares aren't even so much for low income people, as at least in my city low income people already have their fares subsidized. The more people using transit the more likely ballot initiatives pass even if they only occasionally use transit.