Because, acording to Michael Medved, that was the plan:
That theory certainly explains why this year the usual media hand-waivers weren't concerned about the negative effect that negative ads would have on voter turn-out.The advertising avalanche by the Democrats highlighted Romney’s wealth, offshore bank accounts, job-exporting background at Bain Capital, expensive Olympics horse, bad singing voice, mistreatment of his dog, unpublished tax returns, murder-by-cancer of a steelworker’s wife, and general heartlessness and cluelessness. The Obama team knew full well that such messages would never persuade committed partisans to switch support from the Republicans to the Democrats and could even alienate a few of their own backers, particularly women, with their overall nastiness. But if the ugly tone of the campaign kept right-leaning independents or even undecided voters away from the polls then it would be worth it, shrinking the field of potential recruits Romney needed to close the gap and win the election.
In short, the impact of such an unpleasant campaign almost always will prove more damaging to a challenger trying to attract new votes than to an incumbent who needs only to hold on to the bulk of his old base. In this election, the lower overall turnout clearly helped Obama. The president got a smaller share of the vote in 48 of 50 states, everywhere except Hawaii and Mississippi, but he retained enough support in the diminished electorate to hang on to the White House. Lacking any confidence that they could reinspire cynical, disillusioned citizens about the glories of hope and change or the president’s heroic first-term achievements, the Obama high command settled for producing a general distaste for both candidates and even for the political process itself.I visited Ohio the week before the election for a Cleveland town-hall meeting to generate enthusiasm for the Romney-Ryan ticket. The universal complaint from people on the ground in that crucial and advertising-saturated swing state involved thoroughgoing disgust, even rage, at the way nasty attack ads had taken over all the networks and cable outlets. The negativity couldn’t discourage the conservative activists who came out by the thousands to participate in our event, but it’s easy to see how the nasty tone might lead others to turn off the tube altogether and to tune out a political season that had turned unspeakably petty and bitter.
This Democratic investment in negativity may have helped Obama win his bare majority of a radically shrunken electorate, but it also contributed powerfully to the sour national mood and to the widespread perception that the neither of two flawed candidates offered the nation an optimistic path forward. Worst of all, the apparent success of this strategy makes it increasingly likely that future campaigns will follow the Obama example and pursue victory by alarming and discouraging, rather than inspiring and motivating, the already gloomy populace of a preternaturally pessimistic nation.