It’s pretty common sense. Underground breaks cost more to fix because you have to dig the line up both to find the problem and fix it. Above ground problems are visible while underground problems are invisible.

Not to mention that above ground lines have slack between poles while underground lines are pulled relatively taut. Any shifting in the ground is going to result in a break underground before a break above ground. Above ground lines are also better founded because the poles are sitting on foundations or driven deep so their ability to resist ground motion is a bit better while electric lines move with the ground around them pretty much in sync. This is particularly important in seismically active areas like CA but underground infrastructure actually typically performs better in actual seismic events.