Buses have a fundamental speed issue that subways do not because subways do not stop at intersections for cross traffic. At LA's vast scale you need a subway, possibly even express subways.
It's a pain for me personally since a group I'm involved with is active in the San Gabriel Valley and since I don't have a car I have to rely on buses. It's not an exaggeration when I say a trip for me that's 5 miles west would take double the amount of time east. I'm just glad the planned subway to Van Nuys and the San Fernando Valley up north seems to be going well.
LA has pretty much exhausted what it can do with good bus networks though.