Perth's metropolitan region in Western Australia has the largest percentage of underground power in the world. Granted, a large portion is in suburban areas and I suspect entirely in the distribution network not the transmission network. It's climate is very similar to much of California.

Power infrastructure on this magnitude requires an extraordinary amount of capital investment to set up. Who takes on their debt? The operating expenses for asset management and maintenance of a mature network are eyewatering when compared to their underground counterparts as there is a lot more numerous and frequent replacement activity for parts. Pole infrastructure may have a life span of 10-15 years but some parts will be replaced out every 3-5. Before you know it you pay for every pole 4 times over it's life. Now multiply that by several hundred thousand poles in a large network and then you start to get the picture. I'm not trying to be an apologist but I'm surprised from what I hear of American infrastructure that it's not a far more regular occurrence.