Well someone didn't put in a shutoff valve that would work either manually in time to stop the leak, or automatically, or the operator was doing something else at the time.
This is exactly what happens when operators will not do anything except what they are forced to do by regulation. The problem is not with the lack of regulations, it is with irresponsible operators. PHMSA has long been calling for operators to install and use automatic valves for these situations, but with the absence of regulations, PHMSA has no teeth. All they can do is ask.
In this case it appears that having this California regulation would not have helped, since the leak was actually in California, but the pressure source and most of the pipeline are in offshore federal waters. Exactly why the CA Fire Marshall's office returned responsibility for oversight back to PHMSA, because CAFM had no authority in Fed waters and they knew they could not provide effective oversight. Once PHMSA had full responsibility, they didn't have the regulation to force the installation of an automatic valve. The operator could have bridged the fed/state gap, but that unfortunately would have required some proactivity and far too much assumption of civic responsibility by the operator, which is about as rare as hobby horse crap and apparently getting rarer by the moment.