You don't think that people who use services care more about the service than non-users? Whether they're forced into using it or not, the fact they do absolutely makes them more invested in it being good.

Civic and national pride makes citizens (which includes politicians and the wealthy) more likely to care about the actual state of their country. That's what national pride means, as opposed to nationalism, where they are proud without reason. Is public transit guaranteed to be one of those reasons they feel pride or shame? Not at all, but support for it is certainly more likely to come from that than a bunch of nationalists who don't actually feel any shame at failings of the country, of which public transit is currently.

When users of transit are few there is not enough people to care. often the only users are those least likely to be usefuly involved. so you can't get a useful advocate. Even when transit gets support it is from people wanting to feel good about helping the poor - but would never use it themselves and so they want something with no care for quality. They often make alliences with those whose intersts are not for good transit and don't care that the compromise is bad for transit.