Bernie Sanders, who eschewed Super PAC and corporate money, who was essentially blacked out by the media and largely disparaged and dismissed when they did mention him, and was hampered in every way the DNC could think to hamper him without explicitly appearing to favor their queen, is only about 200 pledged delegates behind Clinton.
These facts, in addition to the myriad polls showing how much better Sanders polls against Trump and that Clinton is the subject of an FBI criminal investigation, do not support the myth that Clinton is a stronger nominee to send into the general election.
So when superdelegates, many of whom threw their favor to Clinton before anyone else entered the race or before the first debate (they don’t even TRY to hide the system of cronyism!), claim they are choosing the candidate best positioned to win the general, I don’t believe them for a second. They are playing the game, a game by which you win if you have a villain and you can make everyone live in fear.