The narrator is raised by his grandmother, why?

One such Christmas, the boy and his family go to visit the grandmother and, while there, his immediate family gets in a terrible car crash on an icy road. His parents both die instantly, but the boy is left almost entirely uninjured.

The boy is taken back to his grandmother's house and they stay up together crying and hugging. She tells him that he will live in Norway with her, since she could never leave.