Hillary lost for many reasons:
1) Trying to win a third term for the same party is always difficult (the last to do it was GHWB 3 decades ago, and before him FDR/Truman during WWII).
2) Hillary is unlikable and lacks "that touch" with voters. She would make a better president than she was as a candidate.
3) Taking some voters for granted (as someone said here): didn't visit Wisconsin and went to Michigan only in the last week or two of the campaign despite being upper Midwest states that Trump was targeting (and she lost Michigan in the primary, and so that should've been a warning sign).
4) The terror attacks that happened in the two years or so before the elections and the rise of ISIS.
5) The attacks that she received from the GOP about Benghazi and Libya in general.
6) Questions about her health (especially after that incident on 9/11/2016).
7) Comey's letter 11 days before the elections that sucked the air out of her campaign. She was thinking about winning Arizona before the news broke (and she was there actually).
8) Bill infidelities that Trump used.
9) The vacant Supreme Court seat that made some moderate conservative (who didn't like Trump) vote for him and helped bringing Republicans together.
10) Democrats have an issue with voting: see 2010 and 2014 as examples (without Hillary on the ticket). They were slaughtered in these two rounds (losing the midterm election for the party in power is one thing, but losing that way is another).
11) The attacks on her character: can't be trusted, doesn't understand middle America, elite, etc.
All of that is correct in my view.
But, when I say that Bernie was one reason for her loss, I mean the following:
1) Bernie questioned her judgement, echoing the GOP/Trump
2) Bernie said that Hillary was unqualified to be president. That was a gift to the GOP/Trump.
3) Bernie questioned whether she understands middle America, echoing the GOP/Trump
4) The Bernie or Bust folks (maybe not his fault that they exist, but he tried to bring them on Hillary's train only late in the game). Remember the drama at the beginning of the Democratic Convention with the Bernie people?
5) He stayed in the primary race too long in my view, which led to the following question to be frequently asked: "Why can't she beat a 74-year-old socialist?". "She must be a weak candidate/doesn't appeal to the working class, etc." Staying in the race too late also meant that she continued to work on the primary elections for longer than desirable. Trump was basically done with Cruz/Kasich/Rubio, and started directing his attacks on Hillary. And, sadly, we know that he is effective in attacking others.
6) I know people who didn't vote for Hillary because they thought that, because of Bernie's base, the Democratic party is going too much to the left/becoming too socialist, etc. They preferred a business man over a party with that type of base. Whether I agree with that or not is a different disucssion, but it was a factor.
Bernie Sandres should've understood that the 2016 elections were extremely important and should've refrained from attacking Hillary's character and judgement or calling her unqualified. He also should've left the race when it was clear that she has a big lead (especially among the super delegates) and Trump was in his way to secure the nomination.
And just to be clear: I would love to see a universal healthcare system, I would love to see cheaper college (I see students in my classes every day, and I know that the costs are too high), etc. But, in 2016, there was a more important goal than that.