"There Will Come Soft Rains," by Ray Bradbury, is set in the future. Why doesn't Bradbury place less sophisticated equipment in the empty house and set the story in a war-torn past? How does the theme of the story depend on the setting and why is it a critical element in the story's effect?
Yes, there is no question that the story could have been set in the past, in the aftermath of some cataclysmic world conflict. But that would only dull Bradbury's point. He is sending us a warning: Technology that can be utilized to make things easier for us, do many of our daily, mundane chores. And it is also technology that can be harnessed to bring about vast destruction.
Of course past technologies were used in the service of war. Catapults, bows and arrows, spears, canons, rifles, mustard gas, etc., were all technologies, and they caused horrible bloodshed and death. But the technologies of today and tomorrow may well bring about the total annihilation of mankind.
I think Bradbury sets the story in the future because it makes his point better. I think that a major theme of the story is that technology is getting out of hand. This is more effectively shown in the future.
If all he wanted to show is that we can destroy each other, then the past would have been fine. But he wanted to show that it is technology that is the problem. The house is a perfect way to show this. The thing does everything for people, down to cleaning the dog. This shows how people have let their technology get out of hand.
So, by making it so futuristic, Bradbury makes the story show us more effectively that technology is a danger.