There's still time for him to run in 2016 if the environment is right for that. I'm sure that the decision was difficult for Huckabee to make, but one of the reasons I'm not surprised that he opted against running is the political climate in the country. Our country is very divided right now and while I think Mike would be a great unifier, the attacks that will come at him from the Club for Growth wing of the GOP will not be unifying. He claims that his decision is a "spiritual decision" and as a Christian, I believe him when he says that.
While you may be disappointed in his decision not to run, you should respect that his decision was made after lots of personal introspection and was not a decision that he made lightly.