Gregor the Overlander, Anne of Green Gables, Harry Potter, are all orphans.
In Middle Grade fiction, either kids are orphans or their parents are away and they lead a lonely and miserable life or they have relatives that are horrible to them.
Is this an artificial device to tug at our heartstrings? Is it so cliché that kids are tired of it?
I see four reasons why a writer would make his hero orphan.
Because he could not think of any other device to keep the grownups away. It is lazy writing.
To give a reason for other characters to be mean to the hero.
In order to draw in sympathy from the reader,making the hero look more humble and more pitiful.
Because it gives the kids a reason to live on their own or escape or seek revenge. In brief, it makes it easier to write the kids into situations most parents don’t want their kids getting into. After all, kids that age want to go on an adventure all by themselves. Independence is a major developmental milestone of this age.
Move the child out of the family home into a place where they can be temporary autonomous, like boarding school, camp, runaways, time-traveling, visiting a family member, shrinking the hero, making her disappear from view.
"Lost and Found: The Orphaned Hero in Myth, Folklore, and Fantasy" by Terri Windling