Not because we're smart, but because Iranian president Mahmoud Ahmedinejad ran out of room to maneuver. The deal to ship Iranian uranium out of the country and to process it for use in medical applications seemed full of holes to me. If you don't know how many nuclear installations Iran has, how do you know what amount of uranium shipped out of the country is significant, and what is not?
Ahmedinejad has sworn that nothing would deter him from acquiring nuclear capabilities, and from his statements it's clear those include weapons as well as power plants. Nothing in recent weeks has been a big enough threat to his regime to force him to revisit that fundamental goal.
By refusing the deal, Ahmedinejad has removed an excuse for even more Western dithering and appeasement, and for that we should be grateful.