Because Clinton is the best out of a downright toxic field. Warren is probably their best non-Clinton candidate and she ain't that great if she's trying to appeal to anyone beyond the Democratic base. And who's beyond Warren? Joe Biden? Martin O'Malley? ANDREW CUOMO?
Jim Webb.
But, let's no underrate Hillary Clinton. It's true that she ran a bad campaign in 2008. But, she actually became a good candidate over time and she can apply the lessons of her awful messaging and strategy to 2016. I disliked her back in 2008 myself. I thought her campaign was presumptuous and very bland, almost like her slogan was "vote for me!" I fully expect a different sort of campaign, if only because the mood of the country and the Democratic Party is different. Americans were hopeful and excited in 2008. Now, we're more just looking to someone get the government working at a C- level. We're looking for a contact hitter, it's about Craig Biggio, not Jeff Bagwell to make a mid 90s Houston Astros reference.
As for you people making arguments about how Clinton's foreign policy will doom her campaign, cool your jets. Just because you personally dislike someone or disagree with someone, doesn't mean they're going to lose. That's not how politics works. It's not about your subjective opinions, it's about getting more votes. So, saying "she's a warmonger!" "She's an abortion monger" "He's a peace-nik!" is just a dumb opinion and nobody cares.