Josh Marshall has the clearest explanation I've seen for why, David Brooks's (and Isaac's) advice notwithstanding, there's no way Hillary Clinton is going to take the Huckabee route:
Hillary doesn't want to run for president in 2nd or 3rd gear. It's
beneath her dignity. And I don't mean that sarcastically. It really is.
She's a powerful United States senator, former First Lady, etc. She
wants to win. And if she's still in it she wants to run full bore with
the money you need to run a serious campaign, the crowds, poll numbers,
etc. She's not some Huckabee figure who's going to hang around with
little chance of winning
It really is all or nothing. You've got to convince your supporters,
donors and to at least some degree the media that you're really in it,
and in it with a shot. Otherwise you face the classic problem of a
cascade failure. Poor fundraising generates bad press stories, which
depress turnout at rallies, which create more bad press stories and
eventually no press stories, etc. It's no different from the precarious
position any campaign faces when the odds aren't looking good.
And so we have this vicious cycle in which the longer Hillary's odds
become the further she has to up the ante to keep her candidacy
credible -- in other words, the more forcefully she has to question the
legitimacy of the nomination process and the more aggressively she has
to push the idea that Obama can't win the general election or is not
qualified to be president. (For example, the argument
that the Clinton campaign now appears to be making to funders and the
press is that Obama literally cannot win the general. And thus she's
not only entitled but actually obligated to do whatever it takes to
ensure that he's not the nominee.) Without making real progress on one
of those fronts, the premise of the candidacy just becomes too
difficult to sustain.
--Christopher Orr