Because everybody is paying for it, when they (mostly) don't use it?
I live in a city with decent buses, 3 street train lines and a commuter rail, and use the street lines frequently. Unless your departure and destination are right on the train line, you're still looking at a 1 hour - 2 hour commute with possible missed connections every step of the way. To canvas the city thoroughly you would need buses or trains running every 10 minutes, on every 3rd or 5th block. They would also need to be preferable to the alternative (a personal vehicle with no disruptive passengers, controlled climate, that is available when and where you need it).
Do I love taking the train into the city center for a night on the town, or out to the airport to avoid parking? Absolutely. Will I sacrifice 3 hours out of my day just so that I can say I'm saving the planet? Probably not.
The only way it would work would be some giant weave pattern of train lines where 1 car continues on, one departs left, and one departs right every stop. The budget and logistics of that would make it prohibitively expensive and prone to failure. There are also loads of reasons why people can't take transit (kids, cargo, late night / early morning schedules) and if they're not 80% solved, it will never catch on in the states.