I see the reasons for doing this as arising from the old business justification for being in the center. People on the left may prefer left-wing media outlets to centrist outlets, but they're still an audience for the major centrist ones. Something like that is true on the right, even if it might not be as strong nowadays, since they have a dedicated cable news channel to serve all their needs. Go far enough to one side or the other, though, and the other side will just stop watching you. And of course, people in the middle will go first to the media outlets in the middle. So if your business model is one where you want to compete for as big an audience as possible, you need to be in the middle.
This gives the media two reasons for paying disproportionate attention to Huntsman and people like him. First, he fits the ideology that their business model suggests they adopt -- one that's in the middle. Second, his ability to attract interest from viewers from all parts of the political spectrum far outstrips his poll numbers. I don't find Newt Gingrich particularly interesting, and I wouldn't find Rick Perry especially interesting if he weren't leading the polls, but I found Huntsman's belief in science refreshing enough to be worth a blog post. (As Jamelle points out, Huntsman is not a moderate on other stuff and thinks, for example, that we should repeal health care reform.) So if I actually watched TV news rather than just reading text all the time, I'd be more of a potential Huntsman viewer than a Gingrich viewer.