Well, it's his fault Draco has this bloody sodding Mark on his arm, and so Lucius can wait a little longer. His mother is making a mistake assuming the Dark Lord will free Lucius anyway. It's equally likely it's all a big tease, a motivation that he'll constantly keep scooting back out of reach. In this way, Draco's delay might be a savvy move. If Draco is competent without being too competent, it'll be a mission complete but maybe Draco won't seem to have any serious value. Maybe it'll be the perfect balance of helpful but not crucial, and the Dark Lord's gaze will fall on somebody else.