The Bush administration was not prepared to offer Iran much in the way of incentives, even if Iran cooperated with the international community. That was a problem, because we want to make the choices for Iran very stark between the incentives it would get if it cooperated and the problems it would face if it did not cooperate. The Iranians felt that Mr. Bush was too tied down in Iraq to do anything negative to Iran, and they felt that Mr. Bush would not do anything positive towards Iran, so they didn't see much reason to reach an agreement with the Bush administration.
With the Obama administration, it's much more credible for the United States to be able to say they will provide incentives to Iran if it cooperates, and that's certainly been a factor, but so far, the bigger factor of change with the Obama administration is that the Obama administration has adopted a much more respectful tone and has adopted language that goes much more out of its way to show respect for Iran and its leaders, if I may put it that way.
Many Iranian commentators had said that Bush's insulting tone was a very big problem in relations between the United States and Iran, and, in fact, a very big problem for Iran overall in dealing with the nuclear issue. If that analysis is correct, then this change in tone should help.