I used to take a regional commuter bus between Denver and Boulder, and also occasionally to the Denver airport. Interestingly, the demographics of the regional buses were more like that of the trains - professionals, students, people going to work normal jobs, and people visiting one city or the other for recreation and nightlife.
For me, it was pretty simple: the bus was easier than driving (traffic), took about the same amount of time (express routes required fewer stops), substantially cheaper (parking at my job would have cost ~$15/day), and easy (frequent routes, long hours, accommodations for luggage and bicycles).
Public transit in my current town does not have any of those features. There is no traffic, taking the bus takes longer than driving or riding a bike, parking is free everywhere and the bus costs money, and the bus does not run frequently or on weekend evenings.
In short: if taking the bus results in substantial savings of time, money, or stress, people will take it. Where I currently live, the bus lengthens travel time, increases stress (short hours and infrequent scheduling), and costs more than parking a car; so, people do not take it except out of necessity.