Bernie never appealed to the southern voter. He was filling stadiums up north and was drawing crowds there but his southern strategy failed him BIG time. A fact that one reporter asked a voter in one of the southern states Hillary Clinton won in primaries and the guy looks surprised..."Who is Bernie Sanders ?"
Because as it was detailed that Bernie didn't run as hard in the southern states where he needed to do this. Hillary and Bill concentrated on the south states because they knew those states would be needed in primaries. A fact many of the people remembered how a Bill Clinton presidency helped them in those regions and fell behind Hillary quick.
Bernie never realized that he needed to win the south and just failed.