Perhaps because it raises yet another cronyism argument:
"So, you mean not only was she Bush's lawyer, but she got Cheney into the White House"?
They don't trumpet Miers' involvement in this case because her argument was about as far from an originalist's understanding of the Constitution as possible. The Wall Street Journal had a frontpage story about this case either earlier this week or late last week.
In her defense, she was just zealously representing her client's interests, so I'm not sure how much anyone can take away from the position she took, however.