The same voters that voted for Obama twice in the upper midwest also voted for Trump.
That's probably a bigger issue than the Sanders voters. There are a lot of people out there who are not partisans, pay no attention to politics or the news, and they vote in presidential elections only.
Donald Trump spoke to them. He voiced their innermost resentments. And some, who had fallen for the earlier "Hope and Change" con, also fell for Trump's Bullsh*t. No it wasn't the handful of left leaning primary voters who preferred Sanders over Clinton who might then have voted for Trump.
Face it... It was a perfect storm of f*ck-ups that got us where we are.