Noah interviews various people for their theories as to why this is, but I think the important larger point to recall is that the evidence suggests that candidate attributes in general don’t matter very much in presidential elections. The hard part is winning your party’s nomination, where amidst a field of ideologically similar members of the same party these kind of things can help you stand out.
I think this analysis is generally right, but (admitting I haven’t bothered to read Noah’s article yet, so this might be in there) I would hypothesize that war heroism — or for that matter, simply excessive emphasis on the candidate’s biography — focuses the election too greatly on the candidate and leaves other things, like platform, by the wayside (Kerry, cough cough). One of Bill Clinton’s greatest political strengths was his ability to translate fairly recondite economics into language that made sense to lower information voters. In an election following economic decline (1992), this was far more valuable than prominently featuring biography, which would have eclipsed discussion about the fundamental issues in that election. Certainly in this election, John McCain was woefully equipped to grapple with the economy to begin with and it seems plausibly that the opportunity cost of discussing biography exacerbated an already difficult situation.