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.

The first sign of a peak in cable news appeared in March 2011, when the Pew Research Center released a study that proclaimed, “Though many will remember 2010 as a hard year for CNN, in reality, most cable news channels suffered audience losses.” The able chartists at Pew drew a sad graph of cable news. Combined median viewership for CNN, Fox News and MSNBC during prime time had receded 16 percent, to 3.2 million, that year. Mean viewership had also dropped 13 percent, to 3.3 million, making it the largest year-to-year drop for cable news since Pew started analyzing the numbers in 1997. It also marked the first drop in the median audience since 2006.

The bad news continued through 2011, as cable news viewership remained nearly flat. This was fairly astonishing considering all the breaking news from that year – the Arab Spring, Japan’s tsunami, the killing of Osama bin Laden, the Libyan civil war and the European economic crisis – not to mention the bustle of the presidential campaign.

Among those who noticed that cable news was flatlining was the Atlantic Wire’s Uri Friedman, who surveyed analysts for the underlying reasons in a March 2011 post. The consensus view put the onus on the Web: Now when big news breaks, the polled pundits agreed, the curious go to the Web (often via their mobile device) instead of cable news. Outside the Beltway‘s Doug Mataconis speculated that the potential audience for overtly liberal (MSNBC) and overtly conservative (Fox) TV news had maxed out.

Other possible reasons for the cable news slump is that the three channels (plus CNN’s subsidiary channel, HLN), approached maximum carriage on large cable systems years ago. Upwards of 90 percent of U.S. households already subscribe to cable or satellite TV, and most carry the news channels, so there are very few eyeballs out there that would like to tune in to CNN, Fox News and MSNBC but can’t.