"Sanders did not earn sufficient support from voters to win the nomination."
Of course not. He was doomed on the first day when every single super-delegate, a total of 719, immediately pledged their support for Clinton. That's nearly a third of all delegates. As each state held their primaries (or caucus) the media pointed out Clinton's overwhelming amount of delegates, so why would voters support someone they already thought had lost?
"There are three ways to become a superdelegate. The first is to be elected to public office as a Democratic governor, senator or congressman. The second is to become one of 438 members of the Democratic National Committee as a loyal party activist or powerbroker. The third and most difficult is to become a superdelegate for life by having served as president, vice-president, DNC chair or Democratic leader in either chamber of the US Congress."
https://www.theguardian.com/us-news/2016/apr/19/democratic-party-superdelegates-history-rules-changes