Because it’s Trump’s campaign.
Which has exactly to do with what? The campaign isn’t involved, she may be. She stood up for herself very effectively.
It has everything to do with it. If George Stephanopolus was accused of the same under President Clinton, he would be obliged to say defend his man’s honor and say that he defends his press secretary against a lie. (That is if anyone bothered to cover it, as Clinton hijinx only generate yawns from the media at large)
She did stand up effectively for herself; she is an effective communicator. She shouldn’t have had to do it, any more than you or I should get the “when did you stop beating your wife?” questions.