Because critical infrastructure doesn't get the convenience of shifting the blame. They must account for these risks in advance. Do you know what happens when IT breaks down in a bank? They've got two or more printers that print all new transactions. Database corruption? No problem, we've got the transactions right there. Printer breaks down? No problem, we've got another one printing the transactions right there. Hospitals don't get the luxury of shifting the cause of death from them failing to properly account for risks to a piece of software.