The problem is that diagnosing a bubble is trivial; predicting when it will burst is non-trivial. Both the dot-com bubble and the bubble that preceded the 2007-2008 meltdown were very obvious; less so the precise timing of the crashes because the popping of the bubble rarely depends on fundamentals but rather on the fickle groupthink of over-excitable traders blasted out of their minds on various chemical stimulants, reacting almost mindlessly to various snippets of gossip and surfing on the group's combined emotional wave. Stock markets are very, very far indeed from the economists' "rational person" models so beloved of Nobel prize committees.