The fact that YOU think that Herman Cain is the best candidate does not actually determine whether he will get the nomination or not. If everyone in the country were exactly like you, this would be a valid assumption, but they are not.
Just as the president and VP are from different backgrounds, it makes sense to the parties not to run two candidates from the same minority, or two women on the same ticket.
Cain will be too old or forgotten by 2016.He will get some attention, and some nice memorabilia for his descendants. But no, he will not get the nomination.
This is not up to me. I am just stating the unspoken rules of presidential candidacy. In the same way, a totally competent candidate could not get the nomination if he were too short, too fat or too bald. It would be much harder if he had a vaguely obscene surname, like Lipschitz, Fokker, Nippler or Fuchs.