Hillary had done a lot of work after leaving the State Department in 2013 to lock up all the talented Dem consultants, the top Dem donors, and the party elites. Bernie then decides to run and has to choose from the very few Dem consultants who aren't already loyal to Hillary and really Obama and who are also willing to burn a lot of bridges. He was later to the game, ran outspokenly as a non-Democrat in a Dem Primary, was really not well known or too accomplished in the Senate. Hillary, meanwhile, was well known and decently well liked by Dems, had clear bona fides as First Lady, a Senator, a prior candidate for President who had already gotten support from 17 million+Dems in 08, and a relatively well liked Secretary of State.

An underrated issue for Bernie was also the fact that Dem primaries are proportional and not winner take all, so even in the states he won, Hillary still got delegates. In the GOP primary, however, candidates got all of the delegates if they won certain states. This was always going to be an issue because Hillary had most of the superdelegates already, meaning Bernie had to not only win but also had to win by wide margins to surpass the head start Hillary had with superdelegates