The problem with LA is that mass transit can't work. As cars became more prevelant, there became more sprawl, and mass transit simply couldn't work.
If you take Manhattan as an example, it has under 60km^2 of land and, let's say, 240 subway stops, or 4 stops per km^2 of land. Plus it has major business districts (think "downtown") where millions of people work.
Los Angeles has over 1200km^2 of land, requiring about 5,000 subway stops to have the same density. It's hard to imagine how LA could have an equivalent quality of service to Manhattan while having to be 20x the size.
Counterpoint: Melbourne in Australia has 9,990km^2 of land, and there's very few places you can't get to by public transport. Granted, this includes trains (a few stops are underground, most above ground), trams and buses.