Our narrator's mom and dad don't get a lot of face time in The Witches. They both died in a tragic car accident, which our narrator survived. The only description given to us by our narrator is that 1) his parents were Norwegian, 2) they lived and worked in England, and 3) our narrator felt closer to his grandma than his mom.
After the initial description of his mother and father, we only hear about them on two more occasions, and very briefly – first, when Grandmamma is read her son-in-law's will and, second, when Grandmamma is describing how all witches wear gloves and our narrator says, "Mamma use to wear gloves" (3.11). That's that. Why do you think our narrator doesn't think or talk more about his parents? There's no way of knowing, but it certainly makes his bond with his grandmother seem much stronger.