Continuity mistake: When Harry is viewing Snape's memories, he witnesses a scene on Platform 9 3/4 between his mother and aunt Pentunia, in which Petunia tugs her hand out of Lily's grasp. Her hand is then back in Lily's and she tugs it out again.
Continuity mistake: When Harry talks to Griphook about breaking into Gringotts, Griphook has the Sword of Gryffindor in his hands. When Harry stands up to leave a few moment later, the sword is now beside Griphook's bed.
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.
raywest ★