Probably because the prosecutor has the testimony of numerous individuals (Ari Fleishcer, the Vice President, Karl Rove, the Under Secretary of State) who discussed Plame with Libby before he ever talked to Russert, thus contradicting the testimony of Libby but not that of Russert.
Hmmm... the way I read the indictment on the last count, Libby clearly says he knew she was CIA but didn't want Russert, Miller, or Cooper to know *how* he knew. So he said it was from other reporters.
Is it perjury to mislead Tim Russert?