Because he did so without giving Didio any chance to respond and knowing that, in his current position, he can't.
Plus we're only getting Waid's side of the story, since people seem to be quick to judge that DiDio doesn't tell the truth in HIS comments, why assume Waid isn't slanting the story completely in his favour too. For all we know, he might have been a complete and utter pain to work with.