Three months ago, CNN President Jim Walton told the Wall Street Journal, “I’ve been here for a long time. And I enjoy what I do, and I compete and I plan to keep doing it for a while.”

Today he specified just how long a “while” is. In an announcement sent to CNN staff, Walton revealed his plans to leave his post by year’s end. After 10 years as CNN’s leader, Walton wrote, “I’m ready for a change.”

And so must be CNN, Walton argued: “CNN needs new thinking. That starts with a new leader who brings a different perspective, different experiences and a new plan, one who will build on our great foundation and will commit to seeing it through.”

The news-making event had other hallmarks of a sudden corporate leadership change: It was unveiled just before a slow summer weekend, and it came with prepared statements from Walton’s bosses.

Phil Kent, chairman and CEO of Turner Broadcasting System, praised Walton as a being “smart and steady, tough and fair, business-savvy and respected by his team, and with a track record of great judgment when it matters most.”

And Jeff Bewkes, chairman and CEO of Time Warner, CNN’s parent company, said: “When Jim Walton assumed the presidency of CNN in 2003, it was underperforming and earnings were in serious decline. Since then, he and CNN have tripled earnings, doubled margin and delivered annual growth of 15 percent.”

Missing from those statements is the grim news of recent months. Moments after the decision of the Supreme Court on the Obama administration’s Affordable Care Act was released, CNN for several minutes told its viewers and Web sitevisitors that the law had been ruled unconstitutional; the opposite was true. CNN issued a correction but endured days of pounding from other media.

More difficult to overcome than an on-air mistake are CNN’s enduring ratings problems. Despite its pioneering work in the field, CNN has now solidified a place for itself as third in prime-time viewership, behind MSNBC and Fox News, which leads the charts. As the Journal put it:

The fundamental problem is in prime time. In the year ending in March, Fox News had an average of 1,890,000 total viewers and 428,000 viewers in the 25-54 demographic during prime-time, while MSNBC had 784,000 total viewers and 241,000 viewers in the 25-54 demographic. CNN had an average of 711,000 total viewers and 234,000 viewers in the 25-54 demographic in prime time.

The trick to the ascendancy of CNN’s competitors is no secret to anyone who’s watched a few cable segments. MSNBC and Fox News both rely heavily on their ideological appeal, with much of MSNBC’s programming leaning to the left and Fox’s to the right.

CNN, meanwhile, sells itself as a purveyor of up-the-middle news programming and a first responder of sorts in times of breaking news. Ratings for the network surged, for instance, when news of the Aurora shootings broke early last Friday.

So confident was Walton in CNN’s agility that in June 2010 he pulled CNN out of its agreement with the Associated Press, an outlet that supplies breaking news to many other organizations, including Fox News and MSNBC. Here’s what Walton said at the time:

“We will no longer use AP materials or services. The content we offer will be distinctive, compelling and, I am proud to say, our own. It will provide consumers with the unique news and information experience they expect from CNN. And it will make us more creative, resourceful and collaborative journalists and news professionals.”

AP spokesman Paul Colford declined to address Walton’s departure directly. On the topic of CNN’s break from AP, however, he said, “We’re fully aware of innumerable instances in which they were not competitive because they lacked AP resources and AP content — there’s no question about that.”

Colford cites that famous, contentious encounter between Arizona Gov. Jan Brewer and President Obama on an airport tarmac. AP, he said, had shots of the clash from close range. “They had woefully inferior video taken from an embarrassing distance,” says Colford. AP also got the Supreme Court case right, and early.