The big cities in the U.S. already do heavily subsidize public transportation. None of these cities are full of the sort of voters you imagine to be against this. For example, both New York City and Seattle elected mayors that are self-described socialists. The problem is that the average person in U.S. cities can afford a car which offers better convenience than public transportation and that U.S. public transportation suffers the worst tragedy of commons because it absolutely refuses to deal with people that should be kicked off. It doesn't matter if the bus arrives on time every ten minutes if there's some crackhead that makes the regular public feel unsafe or other riders that are annoying assholes that want to blast their music out of a boombox to the dismay of other passengers. Time, safety, and comfort often outweigh any cost savings that subsidized public transit offers.
Unless I need to get across town, I prefer walking whenever I'm visiting most cities. A weekend bus pass isn't a huge expense, but it's not faster than walking in many cases and I don't have to put up with other riders' asshole behavior. Even though 99% of people are respectful and mind their own business or behave well, it's the 1% of those who don't and the failure of the city to do a damned thing about it that ruins it for everyone else. Europe is t quite at the same level as Japan, but a lot of the shit that makes public transit in the U.S. so undesirable wouldn't fly there. Throwing more busses as the problem ignores the bigger issues and in the future many of the people who rely on busses in the U.S. because they don't own a car will probably switch to robo-taxis because once human labor is eliminated the cost of an individual ride will drop significantly. Make it inexpensive enough and even the poor will take private cabs over the bus because they to value their own time, safety, and comfort when it comes down to it.