Coraline overcomes great odds, defeats the evil other mother, saves her parents, and rescues the trapped souls of three children. Pretty impressive, don't you think? So why doesn't this book end with everyone throwing Coraline a parade to celebrate her awesomeness? Well, Coraline may have triumphed against evil, but she did it alone. And now she has to enjoy her victory alone, too. No one else even knows what happened to Coraline: her parents don't remember their time trapped by the other mother and the neighbors didn't notice anyone was missing.
But even if no one else is paying attention, Coraline is still content. She has learned a lot during the story, and she's much more confident by the end of it, so why does it matter if anyone else knows? She's happy to just drift off into a happy dream, remembering the friends she met in the other world. Coraline is mainly about Coraline herself; so it only makes sense that the end focuses on, well, Coraline.