... People don't like her. Over half of the country is voting over "moral" issues, such as abortion, gay rights, etc. Hillary has strong feelins about those issues which happen to be the exact opposite of those of the people who vote based soley on that issue. Meaning... she would not win.
Anyway, she has stated that she will not run, and the Democrats would probably never choose her as a candidate if she did try.
Note: I have nothing against Hillary Clinton; In fact, I agree with most of her policies. It's just that she would not win.