The primaries in the conservative South are pretty much over. Bernie's chances are much better in the states ahead.
And don't forget that the hundreds of superdelegates can support anyone they want to. These people are Democratic partisans who want their party to win above all else. They can see that Clinton's great support in the South isn't going to help her in November.
I think all those superdelegates are beginning to realize that the national polls that show Sanders' having a better chance to beat any Republican are valid, and that he is, in fact, the more electable one. Then they have a choice: support the party favorite because it's her turn, or support the person who can win.