That's the provocative thesis of this article by Jim Vandehei and Mike Allen in The Politico. And at Slate, Christopher Beam gives a similar answer: "Today's the New York Times A1 piece on Hillary Clinton, 'Clinton Facing Narrower Path to Nomination,' is an exercise in understatement. It nudges the candidate ever closer to the cliff but, maybe because of politeness, or business savvy, or maybe even a perceived need for objectivity, refrains from pushing her over." See also Josh Marshall.
Could it be that the media has kept the story alive? I think that's an overstatement. Of course Clinton can't catch Obama, but it is also true that Obama cannot formally clinch without the superdelegates. So long as Clinton has the hope of an Obama implosion or major gaffe, she has a reason to remain in the race. So don't blame the media; blame the DNC's system for leaving this in the hands of superdelegates---something the DNC surely must reconsider for 2012 and beyond.