Huntsman's departure will have little impact on the race. For some time now, he has had a 0% chance of winning the nomination, and he has been polling at near zero in the remaining states. Huntsman's endorsement of Romney does add a small amount to the impression that Romney is inevitable. Of the candidates that have either dropped out or decided not to run, those who have decided to endorse (e.g. Christie, Huntsman, Pawlenty) have all endorsed Romney. Huntsman no longer being in the race also means one less opponent to attack Romney during the debates.
Huntsman was one of a few candidates to be considered "highly electable." Romney is now the only one of those to remain. Despite his electability and some enthusiasm on the part of establishment Republicans, Huntsman was unable to get anywhere in this race. According to Elephant Watcher's calculations, Huntsman's odds of winning the nomination never exceeded 3%.
Why did Huntsman fail? Partly it's because Huntsman was in Romney's shadow and needed Romney to make a major mistake. The main problem was that Huntsman chose to run to the left of Romney. Candidates have some power over how they're perceived, particularly if the candidate is unknown. Huntsman chose to enter the race by touting his belief in evolution and global warming. This decision, along with the fact that Huntsman was already somewhat vulnerable to being characterized as a "RINO," meant that he would always be perceived as the candidate to Romney's left. There was simply no room. In New Hampshire, voters either liked where Romney stood or they wanted someone to Romney's right. The Anti-Romney voters have always perceived Romney as too moderate, so Huntsman was always off the table. Accordingly, Huntsman has the distinction of being the only candidate in the race who never enjoyed a big bounce in the polls.