Why does Cormac McCarthy choose to not divulge the cause of the apocalypse and what do you think caused it?

I believe McCarthy wrote this story with the intent of making us think about survival. My hypothesis of the cause of the apocalypse would be the devastation of a nuclear war. The threat of nuclear weapons and what would happen to the world globally is no secret to the modern world. Somehow I just don't think he believed that needed to be explained. Possibly, he felt the aftermath would make far better reading than the immediate result the impact of nuclear warheads.