The answer is shown in the prequel: "The Little Mermaid: Ariel's Beginning". Ariel's mother, Queen Athena, was crushed by a pirate ship while trying to save a music box.
Here are more motherless Disney characters besides those mentioned by Bobbi: Prince Adam (the Beast), Prince Phillip, Prince Eric, Eugene Fitzherbert (Flynn Rider), Aladdin, Koda ("Brother Bear"),Tarzan, Jane (Tarzan's wife), Joe (giant gorilla in "Mighty Joe Young") Jill Young (woman from "Mighty Joe Young"), Aladar (iguanadon in "Dinosaur"), Kida ("Atlantis"), Milo ("Atlantis"), Audrey ("Atlantis"), Taran ("The Black Cauldron"), the Wart ("The Sword in the Stone"), Kay ("The Sword in the Stone"), Peyton ("The Game Plan"), Li Shang, Mowgli, Quasimodo, Kuzco ("The Emperor's New Groove"), Todd ("The Fox and the Hound"), Olivia Flaversham ("The Great Mouse Detective"), Penny ("The Rescuers"), and Giselle ("Enchanted").
I watch too much Disney and own each of these movies except for "The Black Cauldron" and "The Rescuers".