You want the answer from a literature teacher?
Disney stories tend to be based upon older stories: folktales, fairy tales, classic novels, etc. Like Shakespeare, Disney tends to pick up a already-classic story, and re-tell it with a new spark, making it all the more memorable; eventually people sort of forget that they didn't create the story at all. These stories rely heavily upon archetypal themes and characters (consistant patterns found across time and across cultures). Archetypal themes include the coming-of-age-story (think Treasure Island), the poor-but-honest youth who "makes good" (think Cinderella), and the damsel in distress (think Sleeping Beauty). Character archetypes include the mad scientist (think Belle's father), the evil stepmother (think Snow White), the monster with a heart of gold (think Shrek -- oops, was he Disney? Well, the point is still the same.) and the earth mother (think Grandmother Willow from Pochahontas).
Disney kept the basic pattern of these old stories -- they kept the archetypes -- thus, we rarely see a loving, capable mother in a Disney story. Those moms who are alive (Mulan's mother) are incapable of providing for their children /protecting them from the world around them. This forces the child /protagonist to face the conflict in his life, rather than being protected at home. Think about it: if Cinderella'd had a loving mother, she would've never gone through years of torment, and she'd never have met the prince.
