Because Harry's parent's death was essential to bring Voldemort down once and for all. If we suppose, that harry's parents were still alive, then Lily would not have cast the spell that protected Harry. Voldemort would have turned Harry into a Horcrux. Lily and James would become evil, while in contact with a Horcrux, 'I was ripped from my body, I was less than spirit, less than the meanest ghost . . . but still, I was alive" that wouldn't have happened, without lily's protection, Voldemort would kill harry at the first chance he got, and the Boy Who Lived would be dead.
