I thought I'd elevate this comment that Steve left a couple of days ago because it lays things out clearly:
The answer is that the reason why Armitage, Libby, and the other leakers weren't prosecuted under the IIPA is that the IIPA requires proof, beyond a reasonable doubt, that the leaker had actual knowledge that the CIA agent's employment was classified at the time of the leak.
To prove that, you need to be able to prove how the person found out about the fact of CIA employment. In the case of Armitage, it was clear that he didn't know; he found out from a document that said nothing about Plame's covert status. In the case of Libby, it was less clear what he knew, but Fitzgerald nonetheless concluded that he couldn't prove anything beyond a reasonable doubt.
The real issue is what Cheney knew and when he knew it. Libby's lies were intentionally designed to keep Fitzgerald from getting a closer look at Cheney and determining what role Cheney had in the leak campaign and whether he knew Plame was covert. That's why the obstruction was a big deal. That's why no one was charged; the IIPA requires that you prove knowledge and Fitzgerald couldn't.
An additional point that's relevant. Most of Libby's defenders -- George W. Bush, David Brooks, etc. -- don't seem to be denying that Libby committed a crime by lying under oath to investigators. They want us to say that, rather, he deserves to be treated very leniently because there was no big deal here. The alleged absence of an underlying crime is key to that theory. The converse theory is that there was an underlying crime and the crime can't be proven because Libby lied to investigators.
If that theory is wrong -- if there really was no crime -- then it seems we ought to get some kind of explanation from Libby as to why he lied. People sometimes do have reasons to lie to investigators other than a desire to cover up criminal activity (hiding non-criminal activity that's embarrassing is the obvious one) but if Libby wants mercy he should offer up a plausible score on this account. But Libby hasn't offered any such story. Instead, he's offered a wildly implausible story -- that he's innocent. Under those circumstances, it's very odd to offer clemency. He's shown no remorse and appears to be continually engaged in a conspiracy to obstruct justice. Maybe there was no crime here; but if there wasn't, then what was Libby doing? He's not even trying to convince us that he had some other reason to lie.