Genuine Ignorance
Somewhat along the lines of the below, I think Atrios goes awry here:
think there's something to the various reasons people suggest - she has the highest name recognition, her support reflects the popularity of her husband - but I really think people are probably wrong to think that these are the sole reasons. Clinton has a lot of genuine support which isn't there simply because of ignorance or nostalgia, and people who don't want her to be the nominee should come to terms with that if they wish to oppose her/support someone else. I'm not saying that support is fixed, just that believe it or not a lot of people support Clinton because they like her.
And I'm not saying people who support Clinton don't like her. I'm saying saying they like her because they're mistaken about her beliefs about American public policy. Most people I've interacted with who like Clinton simply deny that she holds the views that it seems to me she does, in fact, hold. Which, as Ryan Lizza reported is the Clinton campaign's political strategy: "Not that they are trembling in Hillaryland over the fire from the left. The political bet is that, in a presidential campaign, the Democratic base of women, blacks, and labor can be won over in two ways: old-fashioned outreach and stroking by people like Lewis or by the sheer star power of Hillary Clinton."