That's because his 2008 success was more of a cultural phenomenon than a political one (although he and other lefties mistakenly thought it was a political endorsement, a misunderstanding that has caused many of their electoral problems). It was simply "cool" and "in" to be an Obama supporter. Almost every element of the political and popular press hailed Obama to the skies-- NOT to support him branded one as a hopelessly uncool, retrograde, out-of-it loser.
Now, of course, things have changed. And the Newsweek cover is, in a sense, almost a "dog whistle" to those who aspire to be hip, trendy and "with it": It is now officially socially acceptable NOT to like Obama -- and to say so without apology.
For a President who has made it incredibly far on perception, illusion and a certain sort of social pressure imposed by the media and popular culture, this is a disturbing development, indeed: You can now be "cool" AND anti-Obama.