Very typical of children's stories of all sorts -- "orphan fiction" was a really popular genre in the 19th century for both children's and adult books (and continued it the 20th and even now). Part of it was realistic, of course -- kids' parents did die, especially mothers in childbirth. And part of it is the demands of the stories: a lot of the adventures couldn't have happened if the protagonists' mothers had been watching over them and stopping them from doing reckless things. And surely part of it is thematic, too: so many of these books are about young people, often girls, coming into their own sense of identity, moving into their own subject positions, shaping their own understandings of gender. So for psychological reasons, they need to "kill" the mother. Even when the mothers are alive, they are often removed from the action or are ineffectual or overworked or ill or something. (Katniss's mother in Hunger Games comes to mind most recently.)

Just as interesting, imo, are the stories in which the mother is very present -- but that's another whole post.

(Not that I'm telling you anything you don't already know, madam librarian! /g/)