Maybe because she was almost certain that anything he told her was going to be a lie. Draco wasn't exactly known for being honest. Every time she had a conversation with him, even when they were naked in bed and should've been able to be completely open with each other, she never felt like he was being truthful. He was playing some part in an elaborate game that she didn't understand all of the rules to. In the back of her mind she constantly worried that he was just manipulating her and trying to make her a fool. Certainly he wouldn't be the first man to try to do the same. She was exhausted of being lied to. If she allowed Draco the opportunity to explain why her husband was contacting him privately with a magic mirror, she had little faith that she could believe any of it.