In Once Upon a Time, characters' names were kept as they are in the fairy world from Prince Charming to Rumpelstiltskin. But why is Cinderella called by the name of "Ella" by all the characters?
Possibly because "Cinderella" was a mean-spirited nickname given by her stepsisters, and others, especially friends, wouldn't be so cruel. The stepsisters called her that because she was forced to sleep in the cinders by the fireplace, leaving her dirty. "Cinderella" meant something like "little cinder girl".
It's not clear what Cinderella's real name is supposed to have been. But since "Ella" is a modern first name it's a reasonable choice. One example where it's used is the old Ella Cinders comic strip. This name is really a back formation to "Cinder Ella", since it doesn't seem to be part of the original stories. But since those stories didn't give her real name, it's not a bad idea.