If you want another reason, read the work that got him his noble prize and then read his articles.
His work in favor of free trade and analysis got him the award, but for the last decade or so, he has essentially been bashing it (and indirectly, his own work).
I.E. He has indirectly renounced that which gave him credibility (not that it matters, the left ignores the fact that the guy is a former enron economist).
James Taranto frequently reminds his readers that Krugman was on the Enron payroll.