I think that most of his supporters will shift to the first tier candidate Palin, I don’t see Cain taking the Presidency as his first elective office.
As far as you not seeing any reason for them doing so, you must not have been paying attention to the last couple of years of discussion and polls related to that, it has always been know that they compete for the Christian vote.
People who are attracted to Huckabee are not the same kind of people who are attracted to Palin, IMO. He’s a statist, and he probably would have been a Democrat a few decades ago.