> Frequency ranged from 7.5 minutes to 15 minutes, so these came often enough that people could just go out and wait for them, not worrying about a schedule'
If only most cities could have this today
This exists in many cities. See Bus Rapid Transit and many rail lines. Seattle's network has many instances of this.
If I were to speculate, I'd say that perhaps it might only be feasible in small & dense cities with low traffic, as such high frequency requires a lot of resources.
Was born in a small city with frequent public transportation (5-10 minutes for most routes). As I moved to larger & more congested cities, public transport became less convenient and the schedule more erratic.