Joseph Stiglitz explained why he and others got the Nobel prize.

... economists used simple economic models that assumed that information was perfect – i.e. that all participants have equal and transparent knowledge of the relevant factors. They knew that information wasn't perfect, but hoped that a world with moderate imperfections of information would be akin to a world with perfect information. We showed that this notion was ill-founded: even small imperfections of information could have profound effects on how the economy behaved.