It is Sander's fault now?
As I am the one who brought up Sanders, I should respond to this.
Yes, Sanders bears a lot to blame, but no I am not just now saying that it is Sanders fault - I have said it for 8 months. In mid-March I said that Trump would be the next President and that Sanders - both the viciousness of his campaign against HC and the length of time of that vicious campaign as I knew that Sanders would not drop out until the end - would leave HC with lasting damage.
Trump was a terrible candidate who ran a terrible campaign. He spoke to an audience of rural voters and conspiracy nuts who felt (correctly) that they were neglected by the elites of both parties. The conspiracy nuts were neglected because what is believe is insane and at odds with reality. The rural voters were neglected because the solutions to their problems are inherently difficult to solve. Trump spoke of simple solutions. He will disappoint his followers.
For HC to lose to such a terrible candidate took several reasons:
1) Sanders long and vicious campaign against her tanked her favourability rating and made people weary that she could be trusted (again from generally a net +20s to a net -15ish and she never recovered from that).
2) Wikileaks and the FBI hit her where she was most vulnerable. Trust. They didn't have much and if her favorability rating had not tanked earlier it would not have been as damaging.
3) She couldn't hit Trump with what would have been fatal to him. Nailing him on his constant lies and boorish behavior did nothing because that was what was expected of him. If a recent tax return could have been released showing that he was not the massively successful businessman he claims to be that would have been fatal.
4) The polls were underestimating Trump. Some sites were giving HC a 99% chance of winning which was clearly insane, and even 538s prediction of around 65% for HC seemed over-confident to me at the time. This allowed some voters who really did not want Trump to win and were planning on holding their noses to vote for Clinton (who they did not trust) to feel confident enough that it would not matter if they either did not vote or voted 3rd party.
5) Based on internal and public polls Clinton became over-confident in her chances of winning and shifted her campaign funds and events both to help the down-ballot races and to States which were not generally in the running for Democrats. She thought she had the possibility of a massive win instead of just concentrating on the safe win. The result is that she did better in those solidly Republican states like Georgia and Texas etc and ended up losing Wisconsin etc. She and her campaign realized they were over-confident in the last day or two and tried to scramble to save what they had previously considered safe seats. HC generally ran a good campaign, but trying for the big win, to give her a big mandate, was a massive mistake.