I would say mainly because she is on the wrong side of the real and important main issue, which is corporate ownership of our government. The Republican versus Democrat drama is a distraction, and not what’s really happening, no matter how insane and awful the Republicans look. It’s not enough to be not a Republican, not enough to be not a crazy maniac, and not enough to be a Democrat pretending to support the people and to resist the corporations who donate millions and millions to their campaign. Especially not when Bernie Sanders is an alternative.
Because Hillary is part of the corporate corruption problem, and Sanders is not.
Because Sanders is clearly outstandingly honest and for the people and on the good side of pretty much every issue, and Hillary is clearly not.
Because the media has been ignoring Sanders for so long, and acting like Hillary was the only plausible candidate.
Because many in the Democratic Party have been acting like we can’t really choose the honest, pro-people candidate, and need to accept the corporate-funded establishment candidate.
Because Hilary supported things like the war in Iraq, the Patriot Act, the Keystone XL pipeline, the bank bailout, the TPP, fracking, and other industry positions, while Sanders does not.
Because Hilary seems to change her positions and tries to pretend she has the same positions as Sanders, after she finds out Sanders’ positions are popular.
Because there are various stories that show that she is a politician who gives lip service to positions when they are popular, but then doesn’t really support them.
See also:
A great article by a woman about why she’s not for Clinton:
http://www.villagevoice.com/news/feeling-the-yern-why-one-millennial-woman-would-rather-go-to-hell-than-vote-for-hillary-8253224
http://samuel-warde.com/2016/01/video-of-hillary-clinton-blaming-homeowners-for-the-financial-crisis-comes-out/