Mass transit systems need people living near the stops, and places they want to go.
In my european hellhole city planners assume people are willing to walk 200m to a bus stop, 500m to tram/light/metro rail and up to 1km for a heavy rail station.
In that uptake zone there has to be enough people, or enough people wanting to go there for more than a sporadic school service.
Places where people want to go is probably the actual killer. In Europe a lot of cities are built around the main railroad station, with government offices and crucial services nearby, or the railroad is put there because it's where people are expected to want to go. Shopping facilities tend to be crammed together, so it's easy to have a bus route fish people up from the commieblocks, pass by the benefits payouts and end up at the supermarket to spend the money.
In America every settlement off the east coast is an amorphous blob. Shops, jobs, schools and public services are spread evenly. Trips become unpredictable and you're limited to school buses.