Pretty hard to monitor conditions of pipelines when they are underground in an area that gets earthquakes of all sizes all the time.
They damaged it in a repair of a nearby gas pipeline and didn’t handle the damage correctly. Then there was operator error that overpressured the pipeline significantly causing it to rupture. Two human errors.
Pipeline companies do internal inspection of underground pipelines all the time. They send “pigs” through the pipe inspecting for corrosion, wall thinning, weld problems, etc. That is SOP for any pipeline operator.