I don't think high(er) density is necessarily the issue. It's that LA land use/lifestyle/cultural patterns don't really fit with high capacity transit corridors, at least for the non-poor.
For example, there's that vaguely South Florida/Latin American-feeling high density apartment canyon between Westwood and Bev Hills. Not sure if that microneighborhood has a name:
https://www.google.com/maps/@34.0620...7i16384!8i8192
In theory, it would be perfect for high capacity transit. Tall, dense towers along LA's premiere linear corridor, with heavy existing bus service and rail eventually. But that area has very low transit ridership, isn't pedestrian friendly and is unlikely to ever morph into a traditional urban corridor.