The US is huge. There is no feasible way to support public transit for 95%+ of it's land-mass. That's not going to change anytime soon, or ever.
Also, most mass-transit systems in the US operate at significant loss, even with government (ie. taxpayer) funding and collecting rider-fare. A lot of public transit systems are in complete disrepair and are severely lacking. Buses and lightrails are never going to be "cheaper", as convenient or accessible as roads and vehicles are.