Its not any ONE thing.. It it was ONE thing, it would alienate ONE segment.. but the reality is its multiple things..
She's a woman, that's going to energize one segement but alienate another one (I've heard publically many people say they don't trust A woman (not Hillary) to lead the country because they feel A woman stand up to the other world leaders).
Facebook - Yes, its true, facebook doesn't do any filtering, so fake/false/slanderous stories masquarading as "true" are going to alienate many people (in particular those that get their news FROM facebook, which is a large segment of the population these days)
Because she's going to be Obama Lite 2.0 - Yes again, a large segment of the population don't want another 4+ years of Obama... He's done good for a lot of people, but in their opinion "Not fast enough" and it "hasn't helped ME any".. When in fact in a country of 324,118,787 (based on 2016 data) people, you are NEVER going to help EVERYONE.. (everyone, just think about 4 people close to you, they all need your help (sometimes in conflicting ways).. you won't be able to help them all.. you can help solve SOME problems.. but others you may not be because its going to hurt others. Now add in the fact that you have ACTIVE forces preventing you from doing some actions that COULD help (not saying they WILL, but they COULD), and you have former and current legal obligations to others that again, may conflict with your goals. How successful are you.. Now, lets scale that up about 81,000,000 times.. and just see how far you get. Governments (and its impacts are NEVER felt immediately but over years.. especially in a democratic republic.. in a dictatorship you can do whatever you want and its impact (domestically) maybe felt in weeks to months.. but in the US, years to decades.. its like steering a cruise ship... turning on a dime will collapse you, but if you want to turn, your starting that turn 20 - 30 miles out and slowing down so you can resume "normal" speed at the end. A lot of people don't get that.
All of these things COMBINED = the voting public didn't like her.. for a lot of assumed issues, but also some that she actually has..