Here is an explanation by one accomplished professional (Walter Shapiro) who was completely taken in. Excerpts:

About three weeks after Sept. 11, 2001, my wife, Meryl Gordon, and I had an off-the-record dinner with John and Elizabeth Edwards at the Washington restaurant Olives. The dinner was at the blurry intersection of Washington life—ostensibly social (Meryl had bonded with Elizabeth after writing an Elle magazine profile of her husband in 2001) but at its core professional (I was a columnist for USA Today and Edwards had White House dreams). Everyone was in a shell-shocked daze after the terrorist attacks, but my only clear memory of that dinner was Edwards’ palpable dislike for John Kerry, an obvious rival for the 2004 Democratic presidential nomination.

That was the beginning of a political-journalistic courtship that now makes me cringe. With Edwards on trial in North Carolina on charges of violating federal campaign-finance laws—after the disgrace of being caught with a mistress and denying being the father of her baby—I wish I had befriended a comparatively more honorable political figure like Eliot Spitzer or Mark Sanford…

In hindsight, I feel like the jaded city slicker, bristling with self-confidence that he can never be fooled, who ends up hoodwinked by the smiling rural Southern confidence man. Please understand: I did not deliberately put a thumb on the scale when I wrote about Edwards. It was more that I was convinced by Edwards’ sincerity when he talked passionately about poverty and the Two Americas. And I especially believed (because I spent so much time with Elizabeth) the romantic myth of the Edwards marriage.

Many Edwards insiders from the 2004 campaign say the vice-presidential nomination (bestowed by, yes, John Kerry) changed him. The entourage, the plane, the Secret Service detail and the frenzy of a fall campaign all supposedly fueled Edwards’ self-importance and sense of entitlement. But as I struggle to understand my own entanglement with a scandal-scarred presidential contender, I wonder if this arbitrary division between pre-veep Edwards and post-veep Edwards is too glib.

The danger signs and character flaws were always there, and I failed to notice them. I was certainly not alone in my blindness. David Axelrod, for example, was Edwards’ first media consultant during the 2004 primary campaign. Even after Axelrod drifted away to concentrate on a long-shot Senate race for a candidate named Barack Obama in Illinois, he returned for Edwards’ last stand in the Wisconsin primary. I recall running into Axelrod in the Pfister Hotel in Milwaukee on primary day and hearing him say of Edwards, “He’ll be president someday.”…

Yes, the “danger signs and character flaws WERE always there,” and they stuck out a mile. While I hadn’t reached the point of completely dismissing him in print as a phony, you can see my uneasiness with him in this column from 2003:

… There are few things more unbecoming than a millionaire trial lawyer presenting himself to a crowd as the ultimate populist. Huey Long could pull it off; he had the common touch. So did George Wallace. But John Edwards is one of those “sleek-headed” men that Shakespeare wrote of in “Julius Caesar.” He may be lean, but he hath not the hungry look. Mr. Edwards is decidedly lacking in rough edges. Not even age can stick to him.
His entrance was predictably corny. Other speakers had unobtrusively climbed the back steps onto the platform. Mr. Edwards snuck around to the back of the crowd, then leaped out of his hiding place with a huge grin and his hand out, looking for all the world like he was surprised to find himself among all these supporters. He hand-shook his way through the audience to the podium, a la Bill Clinton , thereby signifying that he comes “from the people.” Watch for that shot in upcoming TV commercials.
His speech was laced with populist non-sequiturs. For instance, he went way over the top exhibiting his incredulity at Bush’s “jobless recovery,” chuckling with his audience at such an oxymoron – as though the current administration had invented the term. (A computer scan found the phrase 641 times in major news sources during calendar year 1993 ; so much for novelty.)…

(The point of the column was to say that some protesters who were there to picket Edwards were even worse than he was. But first I had to establish what I’d thought of him. This incident formed part of my better-known “phony” column in 2007, in which I particularly concentrated on a detail I had not used in this piece — because it involved such a subjective impression that I didn’t have the confidence to attach importance to it until I’d had more experience with him.)

I’m not smug for having been put off, from the first time I saw him in person, by what seems to have taken in others. I’m just surprised that they didn’t see it, too.