This may be more of an urban theory question than a history one. I'm a long shot from an expert on the matter, but I'll tell you what I know based on the few city studies courses I have taken.

Los Angeles is a polycentric city, meaning that it has multiple “centers.” As far as urban theory goes, polycentric development was considered ideal for metropolitan regions for a long time because multiple high-density areas in a given region limited travel distances. For example if there are eight major hospitals in a large city a opposed to one, you can just pick the one closest to you as opposed to travelling all the way to the far one. Examples of planned polycentric cities are Toronto and Seoul.

However there's a difference between planned polycentric cities and unplanned ones. In very large cities without planned polycentricity (hope that's a word), polycentricity becomes almost a natural process because of the travel problems that so many large cities have. Los Angeles is an example of an unplanned polycentric city which is why you see these multiple "centers."

Source: An article I read for a course. I took the course over a year ago and no longer have the article. I do have a similar article from Places Journal on Los Angeles and polycentricity.