This is a statistical techniques prize — both men worked on methods for extracting insight from the data history provides us, which generally don’t offer anything like a controlled experiment. As the detailed explanation of the prize (pdf) explains, before Sargent and Sims came along, econometrics consisted largely of estimating models you had no good reason to believe based on identifying assumptions (if you don’t already know, you don’t want to) that lacked credibility. S and S played a key role in developing methods that let the data speak instead.
Full: http://krugman.blogs.nytimes.com/2011/10/10/go-princeton-and-nyu/