I've been wondering this myself. The article doesn't actually say, but what the hell.
DeLong is the only of the four to make a reasonable argument. And I like that he refers to the "book-buying upper-middle class of America", as opposed to the book-reading class, as I believe the latter is a smaller set, and is only mostly intersected with the former (as the smart among us use the library--I'm not that smart, for the record).
I'm perhaps being cynical, but I would have to imagine that for most books the number read:number sold is a relatively small ratio, but for Capital the ratio is dismally so. I imagine that for regular Krugman readers, the allure to buy the book was too much, and they clicked right through to Amazon, or walked over to the econ section of the B & N whose coffee shop they happened to be occupying. Then, they had the sad realization that 500 words with snark and no math is easier to digest than 700 pages with heavy math and--I shall assume, though I've not purchased or read Capital--very little humor, and they laid the book on the coffee table as a decoration.
These four people in the article nobly try to debate the merits of the book, and why it may resonate with so many people. I think they should have invited a psychologist and a stylist, too, if they wanted to get to the heart of the matter.