History and journalism. The story, from a journalist's perspective, becomes too complex if a more complete analysis was included. To write, for example, that the Soviets had a detailed system of approvals which were designed to act as redundancies in the event of errors in launch detection technology or decision-making would muddy the very clear, familiar (and frightening) story that a single man saved us from disaster.

There's also an element of western propaganda and bias here too of course. It was in the interests of western nations at the time to paint a picture of an incompetent and reckless Soviet government and to showcase stories of western-style individual heroics. 

All this said, if Petrov genuinely saved the day, it is absolutely worth celebrating. But we will never know for sure if he did.