CNN was the flag bearer for middle-ground, relatively neutral news gathering in the US, and increased polarisation in US politics after Obama’s election seems to have hurt it the most.
The chart seems to indicate that CNN lost viewers to Fox News in 2008-09. This seems to line up well with the post-Obama election backlash from conservative right, and rise of the Palinistas-Ryanistas / Tea Party movement championed by Limbaugh and supported+tapped by Ailes.
2009-10 saw CNN losing market-share again, while MSNBC held steady – must have been about the time when MSNBC started tilting its programming towards the Democratic left1.
Net result: In 2 years, CNN lost half its prime-time news audience, as viewers on the right and left moved to news networks that echoed their own political views.
While researchers worried about effects of filter bubbles on the internet, the offline news consumption in living rooms was being filtered just as fast.