I wonder if deep down, he thinks he has a chance. It would be difficult to run without a shred of hope, but party prestige is probably reason enough. It's funny to me that MSM journalists must avoid discussing the seriousness of a bid.
Ryan Holiday made the point in Trust Me I'm Lying that the only reason anybody paid any attention to Herman Cain was because he declared early, thus giving them something to do... and by giving them something to do he became a serious candidate. He further argued that without the Internet, nobody gives a shit who's running for president 18 months out.
Cain turned out to be the smartest one of the bunch. I think his whole angle was to sell books, and he sold a lot of them. He never intended to be a contender. If I remember correctly he didn't even set up campaign offices in most early states. He was on a book tour and nothing more, making as much fun of our electoral system as he was himself. Genius.