Justin Therriault credits demographics:
Romney was never that popular in Massachusetts, and there’s nothing in his record that would specifically appeal to the New Hampshire GOP (given that his only notable accomplishment was enacting universal healthcare with an individual mandate). Considering the fundamentals in New Hampshire alongside the cross-tabs of the Iowa entrance poll, Romney’s supposed advantage would be more plausibly attributed to the characteristics of NH voters, especially in contrast to Iowa’s caucusgoers… Romney does stand to do well in NH due to a greater number of college graduates and high-income households in NH than in the rest of the country, both groups which favored Romney in Iowa.