As stated by Jonathan Bernstein in his August 28th op-ed for BloombergView,
"Super-delegates were added in the 1980s for two reasons. One was practical: It was the only way to ensure that those party leaders could get to the convention, at least as delegates. The other was political: Democrats were concerned that their new system didn’t place enough weight on electability, and believed a larger voice for politicians and formal party leaders would tilt the nomination in that direction."There is at least one reason why Senator Sanders has not received as many Super-delegate endorsements, which I would like to briefly elaborate upon here.
Super-delegates consist of United States Senators, Democratic members of the U.S. House of Representatives, Governors, and national Democratic Party leaders. When making the decision regarding their endorsement for president, their primary consideration is the amount of money a candidate can raise throughout the campaign. It's been noted as the deciding factor in 91% of Congressional elections, and Bernie Sanders has overtly made it his campaign's policy to reject campaign financing from corporations. Overall, for Super-delegates it's a safe bet.
That said, the reason most Super-delegates have endorsed Hillary is the same reason Bernie is gaining in the polls. It's not that Sanders supporters hate Clinton. In fact, the recent "Des Moines Register poll of Iowa Democrats revealed that 96% of Sen. Sanders’ supporters are not anti-Clinton. Only 2% of Sanders supporters are doing so because they are anti-Clinton." They just want to elect a president who actively calls for the end of unregulated campaign financing which has definitively biased the legislative and budgetary outputs of Congress and state legislatures.
Your Friend & Ally,
Friendly Neighborhood Democrats