If Bernie wins Iowa and/or New Hampshire, the odds are much more even. Hillary needs an image of being successful to win primaries- many of her voters will abandon her if they think that Bernie can actually win, especially in the general election.
It doesn't help that her campaign, so close to the primaries, has begun attacking Bernie from the left, by trying to create worries about the possibility of people losing health insurance during Bernie's proposed switch to single-payer healthcare. Most leftists already know that they support Bernie and trying to court their vote could very well cost Hillary some of the centrists in the Democratic Party, as well as uninformed voters who don't follow politics.