I bet it's that subways don't compete for traffic, whereas buses have to deal with all the congestion that cars do, in most cases. If my choices are to Drive Myself (my own music, my own pace, no motion sickness, and no uncomfortable staring at the lady across the aisle from me because hers is the only window I can see out of) versus a bus, I'm likely to take the car. This is compounded by not having to __wait__ for the bus schedule.

Taking the Bus + Train from SF to LA area takes longer than driving, and has more stress. Can I stop to use the bathroom, or will I miss the bus? Not an issue if I am the driver. I realize that's not the same as city driving issues, though.