Because they didnt lie to the FBI or the grand jury. Libby did. Judge wouldnt allow them to be witnesses because it wasnt germane to the charges which were did Libby lie and obstruct or not. Its all about the "coverup" not any underlying crime. Libby was not charged with saying whether the Wilson charges were true or not, he was charged with lying about how and when he found out about Wilson and his wife and what he knew about her status at the FBI.
You're not reading before posting. He asked why they weren't put on the stand, not why they weren't put on trial.
Don't let the door hit you in the @ss on the way out.
I understand, but the underlying issue---as is now clear in the statements of the jury---is that Libby was "covering up" for Rove/Cheney, and so at some point the essential TRUTH of the case had to be stated on record, under oath. And a smart laywer could have done that.