This is a matter of personal opinion, of course, but I believe McCarthy's goal in leaving the mankind-endangering event vague in The Road was that the event itself is not the point of the book, and he did not want it to distract from the story. This is one of the bigger failings of the movie version, I think, in that they had to concoct the details and the special effects of the apocalypse in order to satisfy the general moviegoer. Enter the beauty of literature and the compliment of the author not to insult our intelligence.
The point of his story, to me, was how far humanity can fall, and how humans can, in extremely difficult circumstances, lose every shred of decency, or in the case of the man and his son, preserve it. There is both terror and hope inside every individual, and the sheer will to carry on can lead us out of any abyss.
I think this idea is reinforced through the disappointment of the characters (and the reader) upon their reaching the dull, gray and lifeless ocean--that which had been the entire goal of their harrowing journey--and this once again focuses the attention of the reader on the characters themselves, their humanity and their struggle to maintain it.
On page 1 of The Road, McCarthy only gives the reader an ambiguous and detached account of the apocalyptic event. As is his style, he doesn't even use a complete sentence:
The clocks stopped at 1:17. A long shear of light and then a series of low concussions. He got up and went to the window. What is it? she said. He didnt answer. He went into the bathroom and threw the lightswitch but the power was already gone. A dull rose glow in the windowglass. He dropped to one knee and raised the lever to stop the tub and then turned on both taps as far as they would go. She was standing in the doorway in her nightwear, clutching the jamb, cradling her belly in one hand. What is it? she said. What is happening?
First of all, an apocalypse is unlikely to happen. Even more unlikely is that a random man and wife, miles away, would know its source or cause. Because he frames the novel from the father's limited perspective, we cannot know the cause of the concussions because he does not know. The event is presented in flashback, as if part of the father's distant memory. Also notice the dialogue the between husband and wife: "What is it?" and "What is happening?" Even the line of questioning is ambiguous. This couple is not close to the epicenter of these concussions and obviously don't know the details.
Second of all, The Road is a kind of allegory more than a science fiction novel. McCarthy doesn't even supply names to his main characters: they are simply man and boy. As such, it doesn't matter what happens, only that it does happen: the journey is more important than the origin of it. To supply some science fiction explanation of the event would be a wrong move; it would sound cheesy and false, like some 1950s B-movie. Imagine how the sound of a meteor impact, nuclear explosion, or radioactive outbreak would devalue the seriousness of the allegory on the first page?
Isn't it better to let the reader infer the cause? As I said in the eNotes introduction:
The novel can be read in a variety of ways. The Road is perhaps the most chilling commentary of the post-9/11 world. The post-apocalyptic setting plays upon the public’s fear of terrorism, pandemics, genocide, and weapons of mass destruction.
But, 50 years from now, these fears no doubt will be replaced with new ones, and rather than date his novel, McCarthy wisely avoids specifics.