This is not quite true, plenty of new-build metro systems continue to use a 3rd rail DC traction system, esp. in China to reduce the size and cost of tunnels, and lower the weight of train equipment for better acceleration (with DC you don't need an onboard transformer).
For Electrolink, the system would largely operate above ground so tunnel clearances are not a major issue (yes, i know there are a few tunnels on the system but not miles and miles of tunnel). Acceleration is less of an issue due to widely spaced stations. The substations can be smaller and further apart than a 3rd rail DC system, all things equal, and real estate in California is expensive. Lastly you want inter-operability with the HSR buildout from CAHSR and Brightline, at least on the Antelope Valley Line and LOSSAN, maybe on the San Bernardino Line if Brightline ever makes it to Rancho Cucamonga. So OCS is the clear winner - except some Californians think it's ugly.
