Essentially he was the runner up in 2008, he and Romney. Romney had his second shot, and took the brass ring. I'm sure Huck thinks he can win it, too, this time.
I think a lot of folks here are mistaken in believing that he's being paid to run. He seems to have the requisite over-sized ego to think he can win, nothing more sinister than ambition.
Looking back at 2008: the four candidates who actually won primaries were McCain, Romney, Huckster and Ron Paul. If you were a conservative voting in the later primaries (when some of the minor candidates like Tom Tancredo had already dropped out) who would you vote for?
To me, this explains a lot about why McCain won. And it does not bode well for Huckster in 2016, as there will be plausible conservative candidates for voters to choose instead of him, not just two RINOs.
Slight correction: Ron Paul won delegates, not States, and was not really plausible, though he stayed in till the bitter end. So he was available for Conservatives to vote for, but probably not attractive to many Conservative voters due to his unorthodox views on many topics.