He shielded Armitage from blame (Armitage was the one who let it slip that Plame was the agent) and allowed Libby to twist on the vine.
He refused to budget on the issue and will not give up his pride anytime soon.
I am also guessing that Powell is very resentful of the fact that he put his credibility on the line by going to the UN and essentially telling the world that the US had solid evidence of substantial WMD in Iraq. When the WMD turned not be there (more or less, at least not anywhere near the extent that had been thought) and then the war turned into a fiasco, Powell likely felt humiliated and harbored a grudge against the Administration for having nudged him into doing it.