* if a bus ahead breaks down, the bus behind can still go around it, so the line's not totally clogged up
* busses are quieter going through neighborhoods (have a friend who lives by Duboce Park and it rumbles)
* you can share the road with regular automobiles, whereas rails depend on the implementation (yes most of the ones in SF do as well)
* easier to turn a bus around than a train at the terminus
An actual subway could be quicker, but again that requires money, cooperation from the neighborhood, and time.