Sharpton isn't so willing to play lapdog. In the past, he's turned on the party and endorsed Republican candidates. Get it? Sharpton won't sell out. Given the appalling lack of black senators and congressmen, that makes Sharpton black America's most viable mouthpiece.
Sadly, our civil rights leaders refuse to back him because they've been conditioned by Jackson's success to believe that a black candidate needs the party's blessing. They refuse to endorse a more ideologically pure leader like Sharpton because he is not so eager to placate the party (and also because they nurture their own dreams of currying favor with the party and supplanting Sharpton as the voice of black America).
If instead our civil rights leaders united behind Sharpton, they wouldn't have to prostrate themselves before the Democratic Party. They would have enough of a unified following to force change - or at least a genuine give-and-take - on issues of real importance. Then Sharpton would be something rather extraordinary - a black leader who didn't have to sell himself out for white support.