By playing nice, each candidate is acting in his own rational self interest — but it just so happens to be an existential threat to the group, collectively.
Individually, this makes perfect sense. Some of the candidates, by now, know they cannot win. As such, they have little incentive to attack Romney. (Perhaps he will give them a position in his administration if they help him? — Why ruin that? Or maybe he would counter-attack them and make them look bad if they criticize him? …. Or maybe they just want to be thought of as “nice”?)
Meanwhile, the candidates who think they can win — Rick Santorum and Newt Gingrich — probably believe their best shot at the nomination is to finish second in New Hampshire. And while going “negative” in a debate may hurt Romney, it would also tarnish their reputation, as well.