Not directed at folks on this forum, exactly, but in regards to general US sentiment much of the reason is that she's been under a series of fairly dedicated media attacks for something like the last decade, basically unceasingly. She's had shit made up about her, she's had shit blown tremendously out of proportion, she's had almost everything constantly shown in the worst light possible, she's had almost everything decent she's done drowned out in shit-slinging, attempted to be reframed as a negative (see mentions of flipflopping, only caring about present political prudence, etc.), or just barely covered, just as a non-exhaustive list. It's a case where I literally can't recall any american politician get as much negative sentiment/media attention as hilary does, over a period as long as she has.
She also has legitimate criticisms to be made for some of her previous actions! They're just pretty much never treated the same way those kind of criticisms are for other politicians with similar (or greater*) fuckups. And she's certainly more centrist than bernie, of course, it's just that, y'know, so is pretty much the entire US political system. It's just somehow a cardinal sin for her, at least vis a vis the primary season. Probably mostly because there's only two dem candidates, but still.
*I had a lovely conversation recently where someone railed quite vigorously over the four military folks that died at benghazi, and then blew off the 2k+ US military dead (nevermind the civilian casualties, of course) from Bush's bullshit, just as an example.