My understanding is that building subways in LA is costlier and more labor-intensive than in a lot of the cities you mentioned, but the subway system is currently undergoing a massive expansion in LA.
The main difficulties are the fault lines: Subway lines have to be meticulously planned to avoid the dozens of faults as well as other areas that are susceptible to major movement.
The main difficulties are the fault lines: Subway lines have to be meticulously planned to avoid the dozens of faults as well as other areas that are susceptible to major movement.
http://www.cccarto.com/faults/la_faults/