Because a racist right-wing blogger, hell-bent on stirring up racial hatred, posted an edited version of a March speech in which Shirley Sherrod spoke about a very touching, helpful incident that happened 23yrs ago, and the FOX' we-hate-Obama gang of Beck/Hannity/Gingrich/O'Reilly ran with it as if it happened recently, Shirley Sherrod was asked to resign her fed gov't job - nobody can confirm it was Pres Obama who asked for Sherrod's resignation.
There will always be those who can't stand the fact that black people have the same rights as everyone else - it will always stick in their throats and a black, highly educated man in the oval office is more than they can handle!