Trivia: Hermione's middle name appears in Dumbledore's will as "Jean." According to J.K. Rowling, her middle name was supposed to be "Jane," but in a subsequent interview, Rowling commented that she changed it because she did not want Hermione to share her middle name with Dolores Umbridge.
Trivia: Because the difficulty of translating "Deathly Hallows" to other languages, JK Rowling created an alternate title for the translations: "Harry Potter and the Relics of Death".
Trivia: J. K. Rowling considered two alternate titles for this book: "Harry Potter and the Elder Wand," and "Harry Potter and the Peverell Quest."
Trivia: In the Norwegian translation, the name of the chapter "The Flaw in The Plan" has been translated in a way that creates a double meaning, one hinting to the fact that the Elder Wand has an important role in the chapter. The chapter title has been translated to "Streken i regningen", which means something like "The line in the calculation" (A Norwegian expression that means the same as "The Flaw in The Plan"; a problem you haven't noticed). And the Elder Wand is symbolized by a line.
Answer: There's no known explanation. For one, it serves the book's plot to keep Lucius and his family alive throughout the series. For another, Voldemort liked mentally torturing his victims, stretching out their fear and misery for as long as possible for his amusement. He may also have felt that the Malfoy's usefulness had not yet entirely run out.
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