MIT just did a similar type of project called Electome for election Twitter. Although it’s limited just to tweets, it appears to show a similar pattern. The left-right news disconnect could very well could be due to certain organizational policies that discourage (or outright prohibit) reporters/editors linking to external news actors.
From what I’ve personally seen content-wise, The New York Times seems to be one of the worst in this regard — many of their articles are just a carefully groomed sequence of self-referencing internal URLs. So I think it’s part org policy, part journalistic culture, part not taking the right seriously/editorial hubris, and part lack of a clear picture of the digital/social competitive news landscape.
Just some thoughts — I’m sure there are many reasons.