Because he's one of the of the most respected scholars in the field of Economics?
He's likely devoted thousands more hours than us pondering economic matters. I'm just looking through his wiki here... published hundreds of scholarly articles and 20 books. A professor of Economics at some of the best universities in the world. He's even received a Nobel Prize.
How can we fault someone for taking Paul Krugman seriously on economic matters?
The Beatles had Brian Epstein for example (aka The Fifth Beatle)