Because that's all they have. There was no collusion. There was nothing found on Trump. Zip, zero, nothing. So, they have to focus on Trump getting mad and wanting to get Meuller fired. Roby tried to put a stop to that but needed to get to her questions. Ouch, FNC just said Mueller has lost it.
All they have is hammering McGahn and what he states was said and done. One person. How does the President defend against that?