Because a lot of women died in childbirth back then.

Because it gives her a pleasing aura of tragedy.

Because the hero’s mother is also dead, and we’re planning to have them bond over that.

Because she’s supposed to be an extension of her wise/powerful/evil father, and adding extra family members would complicate that.

Because her storyline is about growing up and finding her place in the world without relying on parental figures (her dad doesn’t count).

Because her father is deeply unstable, and the audience would never buy that he could either keep a marriage together or win custody of children in a divorce.

Because if she wasn’t the only thing in her father’s life, then his locking her away and discouraging her from making any friends her own age might seem a bit unreasonable.

Because we want the male characters to be the ones teaching her about the world.

Because we only have enough ideas for one female character, and we can’t manage another.

Random vampire attack.  Could happen to anyone.