Arkansas has always been underpopulated--it's only slightly smaller than Illinois but has less than one-quarter as many people. Maybe Huckabee thinks that if Americans won't come live there, that's another job for illegals to do "that Americans won't do."
Of his exact position and reasoning, I can't say. I just remember someone on some thread railing about his immigration stance.
Arkansas' agricultural processing industry certainly employs a large number of illegals.