One possible answer is Lucy's natural sexiness. She's innocent and virginal, but she's naturally much more sexy and voluptuous than Mina is. If Mina is everyone's mother, Lucy is everyone's wife: at one point, she says to Mina, "Why can't they let a girl marry three men, or as many as want her, and save all this trouble?" (5.11). Though Lucy wouldn't mind marrying all three of the men who propose to her, she chooses Arthur, because she loves him best.