Making trains go where people aren't relieves the pressure on ever increasing prices in populous areas. People will have a realistic option of living in cheaper areas.

AND it allows real estate speculators to get a much higher rate of return when they build up cookie cutter neighbourhoods in cheap land that is soon to be linked up by rail.

Both a good and a cynical reason for this behaviour, which is why I don't think you'll ever see it change (no matter where you are).

It's not an either-or proposition - the local subway systems in both LA and the Bay Area are undergoing very large expansions at the same time.

Money is being spent on local transit improvements in the LA and Bay Area as part of the High Speed Rail project.