Hillary Clinton probably has the highest negative ratings of any national politician (with the possible exception of George W. Bush). Polls indicate that something approaching an absolute majority of the American people would "never" vote for her for president. It's not the fact that she's a woman; the polls also indicate that people are less opposed than ever to the idea of a woman president. But Hillary comes across, at least to many people, as calculating and fearsomely ambitious. She is no longer (if she ever was) the valiant little wife "standing by her man." She is widely regarded as cold, hard and determined to have her way. Not for nothing do Republicans call her "the Ice Queen."
What, then, is to prevent the Democratic convention from passing her over and nominating somebody else? In theory, nothing; but that's not the way things necessarily work. The Republicans in 1964 knew that Barry Goldwater was a long shot, but they nominated him anyway. So, too, did the Democrats in 1972 pick George McGovern, even though most of them knew full well that he was far outside the mainstream of American political opinion. There are times when a personality, or an idea, so overwhelms the competition that resistance seems not only futile, but ridiculously stubborn, even when it's based on common sense.
So the Democrats may well have Hillary in their future, regardless of her high negatives, unless one of her possible competitors can bring her down in some of the major primaries.
And nobody is within sight of that achievement yet.