Partly because the writers haven't given Mary O'Donnell anything to do. She was positively wimpy in this last episode. Rusty was shouting at her in her office, and she kept saying "Let me explain" but he kept on shouting and drowing her out until Provenza stepped in and put an end to it. It needed a strong male voice to set the kid straight. Personally, I was hoping Rusty would meet his father, take an instant liking to him, pack his bags, and leave the series forever.