So, we've got kind of a three-part ending here. We start with (1) people abandon Mars and then hit (2) the Earth is a wasteland, and then finally comes to (3) a few people escape back to Mars. Let's take these one at a time.
(1) In "The Luggage Store" and "The Off Season," the settlers learn that war is breaking out on Earth. Can't miss it! Pretty much everyone decides to abandon Mars and head home.
Honestly, we don't really get this. Are they going back to help the war effort or to be with their families? (Remember, these stories were written shortly after World War II, when everyone in America was supposed to help out the war effort in some way.) Or are they going back because they don't want to miss out on the excitement?
Even when the war is over, in "The Long Years," Captain Wilder wants to return to Earth to see what it's like (52).
(2) The Earth is a wasteland.
Okay, "There Will Come Soft Rains" may make you want an awesome automated house, but the awesome house really isn't the point. During the 1940s, people would also be really worried about nuclear war and annihilation because of that little problem with the nuclear bombing of Japan. In fact, the 1940s saw lots of novels and short stories about how a nuclear war could lead to the end of the world.
So this part of the ending is very much a reflection of Bradbury's historical moment. For the first time in human history, people actually have the technology to kill every single person on Earth. Pretty scary stuff.
(3) We have no idea how this will end up, but the very last story in the book is about people returning to Mars in order to get away from the war. Now this seems totally reasonable to us—why didn't everyone come to Mars before?
What stands out to us here is the point-of-view: we see this story through the eyes of a kid who is old enough to cry about the tragedy of the situation (95), but also young enough to see the possibility of this new place. Will Mars be humans' new home? And will they revitalize and settle it—or will they just blow themselves up again?