The main reason many Disney characters didn't have living mother's was because 1, their original fairytales stated that the mothers were either already dead or were killed. (Cinderella versus Snow White. Cinderella's mother already being dead, and Site White's mother being the Evil Queen and was killed at the end of the story.) And 2, Walt Disney's own mother was killed in the house that he had bought, due to some kind of problem the house had, and he felt personally responsible for it.
And when they don't see their moms until they're a teenager like Hercules, those are the characters who need to learn who they are before returning home. So They can find the path that is right for them.
Sometimes they kill brothers or grandmothers instead. Or fathers, as was mentioned already with Mufasa.
They haven't always killed family members of the hero. Zootopia didn't, and neither did Sleeping Beauty, Hercules, Tangled (unless you count Gothel who was also the villain and not Rapunzel's real mother), or Wreck-It Ralph (though they did program a supporting character with a backstory of her fiance being eaten). I haven't seen it in a while but I don't think Peter Pan did either. I'd say Mulan, except maybe it counts that Shang's father was killed. But, most of them did, either by having some dead by the beginning, or killing them either onscreen or offscreen in the course of the story.
I still think that, between villains and family members of the hero, they stopped killing the wrong one.