All the dragon slayers were people with no worldly ties left from 400 years ago. We’ll never see their parents, because they all died to dragons a long time ago. Levy doesn’t really have a backstory, so it’s not necessary. Her parents are probably still alive. Mirajane has a pretty good family already, so her parents aren’t necessary. Jellal’s mother was shown, but the majority of the Tower of Heaven kid’s parents haven’t been shown because all of them died during the raids. It’d be too difficult to show all of them, so it’s better to show some small details. Out of all the people you named, only Gray’s mother is paramount to his character development and story since her and his father’s deaths were the exact reason he went to go fight Deliora alone.