Rail vs. Bus has been a consistent battle since completion of the Blue Line in 1990. By connecting Long Beach to Downtown L.A., the transit planners resolved some of the commuter headaches of the 710 and 110 freeways. In 1993, the Red Line opened it's first leg from Downtown to MacArthur Park and exposed all the difficulties with building a heavy rail line (a.k.a. subway) in a city as developed and busy as Los Angeles. Other communities began clamoring for their own rail lines and the expansion continued with the Green (traveling on the 105 fwy.), Gold (Pasadena to Downtown and now to East L.A.) and the faux Orange (technically a Bus Rapid Transit system, but close enough for the Valley). Now Metro officials have declared that Crenshaw Blvd. should receive it's only Light Rail from the Expo Line which is currently underway to the existing Green Line.
I am personally not 100% convinced that rail is the best way to spend transit dollars in a city as sprawling as Los Angeles, but it is understandable why it continues to win the battle vs. bus debate.