They have to take him out early.
They cannot risk not having the melanin based criticism shield that 0bama has being swept aside.
This cannot be emphasized enough. When it comes to "electability", Cain has a huge advantage over all the other candidates, Obama will not be able to effectively use the race card against him, and Obama never would have got elected in the first place if the election was race neutral.
Beyond that, picture the effect if those racist Republicans (or so the democrats claim) nominated a black man for president. That would fly directly in the face of one of the most oft-repeated democrat talking points.