In fact, not to get off-topic, does anyone know why light-rail was chosen for the Blue and Green lines over HRT? Was it purely just because of cost?
Cost, construction time, politics and timing.
The RTD since the 1960s had anticipated building an all heavy rail system (hence the name "Rapid Transit District" which was never realized until the end of its existence, ironically), inspired by SF's BART.
The lack of political will and funding (the subway was primarily funded by federal funds, which could be delicate depending on who's president and who runs congress) kept that system from becoming a reality until the 1980s.
Around that time, Calgary and Edmonton, Canada and San Diego started the light rail revolution. Supervisor Kenneth Hahn rode the system in SD and took political leaders to the top of Los Angeles City Hall, pointed to the southeast and said, "See that line over there? That used to be the Red Car line to Long Beach. We can build a light rail system for less cost and less time than the subway!" Unlike the subway, the LRT was funded primarily from local funds (The initial 1980 Prop A 1/2-cent sales tax) and is subject to much less politics and bureaucracy.
The Metro Rail was the RTD's project, but the Blue Line (and later the Green Line, which was incorporated into the Century Freeway project) was the LA County Transportation Commission's baby. The two entities both were in charge of transportation in LA County, in different and overlapping ways.
Truth be told, the RTD's subway broke ground in 1986, and the first 4.4 miles of it opened in 1993. The LACTC's light rail broke ground in 1987 and the first 19 miles of it opened in 1990 (it was opened in its entirety in Feb '91).
Meanwhile, the entire 17 miles of the subway didn't open until 2000. So that's 14 years to build 17 miles of subway, vs. 4 years to build 22 miles of light rail.
Basically, that's why local governments and planners look to LRT first around here. Even with cost overruns and construction delays (coughExpocough), it's still cheaper and faster to build.
Not advocating against HRT, heck, I'm probably one of the few people here who lives walking distance from the subway and use it all the time, but just presenting a historical truth.