Reading a lot of post-mortems on the Hillary campaign, but it usually comes down to tactics and styles and blunders. All of those things are important, i'm sure, but the main reason, which is generally not discussed at all, is this (printed in Feb 07):

One of the most important decisions that Senator Hillary Rodham Clinton made about her bid for the presidency came late last year when she ended a debate in her camp over whether she should repudiate her 2002 vote authorizing military action in Iraq...antiwar anger has festered, and yesterday morning Mrs. Clinton rolled out a new response to those demanding contrition: She said she was willing to lose support from voters rather than make an apology she did not believe in.

"If the most important thing to any of you is choosing someone who did not cast that vote or has said his vote was a mistake, then there are others to choose from," Mrs. Clinton told an audience in Dover, N.H., in a veiled reference to two rivals for the nomination, Senator Barack Obama of Illinois and former Senator John Edwards of North Carolina.

Her decision not to apologize is regarded so seriously within her campaign that some advisers believe it will be remembered as a turning point in the race: either ultimately galvanizing voters against her (if she loses the nomination), or highlighting her resolve and her willingness to buck Democratic conventional wisdom (if she wins).

(While a lot of bloggers have commented on the lack of discussion of the war as a factor, my post is basically a paraphrase of Jonathan Schwarz's on This Modern World).