Because he's the only Republican
At NRO, Stephen Spruiell says:
Just off the top of my head, and in no particular order:
Former White House spokesman Ari Fleischer testified that he told John Dickerson (then with TIME magazine) about Valerie Plame. Dickerson denies this.
Fleischer also testified that he told NBC’s David Gregory about Valerie Plame. NBC Washington bureau chief Tim Russert later denied that Gregory ever received the leak.
Washington Post reporter Walter Pincus testified that Fleischer told him about Valerie Plame. Fleischer denies this.
Bob Woodward testified that he had discussions with Pincus about Valerie Plame. Pincus denies this.
Russert first told the FBI that he couldn’t rule out the possibility that he discussed Valerie Plame with Scooter Libby. He later testified that he could.
NBC’s Andrea Mitchell has yet to testify, but she first said publicly that everyone knew about Valerie Plame prior to Robert Novak’s column. She now denies that she herself knew (and successfully resisted a subpoena).
And now, today, this:
On Tuesday, Jill Abramson, now The New York Times' managing editor, was called first by the defense [...]
Abramson was asked about former Times' reporter Judith Miller's testimony (she had claimed this for a long time) that she had asked Abramson in 2003, when the latter was Washington bureau chief, if she could write a story related to WMD and her recent talks with Libby.
Abramson, as she has done in the past, denied it.
Remind me: Why is Libby the only one on trial for perjury?