Danny Yadron at the Wall Street Journal's Washington Wire blog asks that question, and includes NOM Chairman Maggie Gallagher's comments:

Former Massachusetts Gov. Mitt Romney, Rep. Michele Bachmann (R., Minn.) and former U.S. Sen. Rick Santorum have signed a pledge from the National Organization for Marriage to oppose gay marriage at several levels of the federal government.

But former Minnesota Gov. Tim Pawlenty, who has sought to burnish his credentials as an evangelical Christian, declined to sign it, possibly putting him at odds with the social conservatives he is trying to court in Iowa. The National Organization for Marriage is running a four-day Iowa bus tour leading up to the Aug. 13 straw poll in Ames.

“Pawlenty’s communications director Ann Marie Hauser, personally informed me on Tuesday that Tim Pawlenty would not sign NOM’s marriage pledge,” said Maggie Gallagher, chairman of the board of NOM. “Like many people, we are scratching our heads wondering why Gov. Pawlenty, who has been a champion for marriage in Minnesota, would not commit to doing so for America.”