<Snipped quote by FantasyChic>
There's really two reasons for it, too!
A) supporting casts, either they're hard for the writer too pull off. Maybe the writer feels like they distract from the "plot" and if every character has 5+ supporting members all the time, why would we play with each other when they could just play with their own characters all day?
and B) Because we, as writers are all hopeful that we can be good people (Or just, like, people.) Despite tragedy, and it's nice to see that in characters we create. And of course, that characters who thrive despite tragedy is cool!
I'm calling it the Disney Syndrome. Why? Because I can.
Dead parents are the absolute easiest NPCs to ever play. This is a true fact. Unless they wind up haunting the character, and then everything goes crazy.