Avinash Dixit states the bloody obvious:

In the last 10 years, Krugman has achieved fame in a much larger arena with his columns in the New York Times. These offer strong views on economics and politics, and they have been harshly critical of the Bush administration on most issues. It is no wonder that they attract adulation from readers who share his views on these matters and hatred from the other side. The former delight in his Nobel Prize, and the latter are shocked and dismayed by it, but both these reactions are mistaken. The prize has nothing to do with the Op Ed columns and would have come to Krugman just the same if he had never written a single one of them. The prize celebrates his achievements in science, not in the policy arena.

Except it’s not so bloody obvious, it seems: “You don’t get the Nobel Prize in Economics for writing newspaper columns (as I’ve been trying to explain to my mother the last couple of days).”