You need both, but it's not exactly realistic to have metro lines everywhere. The west side is pretty dense and has a lot of commercial/business activity so the value per dollar is high. By comparison the east side is super spread out and has a low(er) residential density so the value is far lower. Not to mention a good chunk of the residents on the east side aren't in favor of metro lines in their neighborhood (the stalled Whittier extension of the E line).

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.