I don’t think we can point to one single reason why she lost and say that’s the definitive answer. Would she have won if she just campaigned more in the upper midwest states she lost? Perhaps. Would she have won if Comey hadn’t dropped his email bombshell late in the game? Perhaps. Would she have won if she were more to the left or more to the right? Who knows. I am a Bernie fan, but unlike most Bernie fans I am not 100% convinced he would have won either, a lot of this is Monday-morning quarterbacking-- how would Bernie have performed in debates against Trump, what kinds of scary talking points would Fox News create about Bernie, and would most of middle America buy into them or dismiss Fox and trust Bernie? I have no idea.
The American electorate is a chaotic mess, and it’s impossible to perfectly predict what will happen in a chaotic system 100% of the time.