CNN’s rotten ratings have grown only rottener. The Time Warner-owned news network drew fewer prime-time viewers last week than any week since September 1991, the New York Times just reported. But CNN isn’t the only network riding the down escalator when it comes to ratings. Over the same week, Fox News Channel attracted its fewest viewers in the important 25-to-54-year-old category since July 2008, the Times added. * But CNN isn’t the only cable news network in the doldrums, according to year-by-year data. Various observers have blamed the viewership downturn on the lull in the 2012 campaign, on viewers defecting to the season finales on the entertainment channels and on the lack of breaking news. But I interpret the falloffs as fresh evidence that the audience for cable news has peaked.