I don’t know why you’re surprised or confused about this. Bernie was an independent and not even a member of the DNC until just before the election.
He was an unknown. Hillary has been a part of the party for decades. That’s like complaining that the DNC didn’t throw full support behind a Green party candidate that switched just before the election. That’s not bullshit. That’s just how politics works. Bernie didn’t lay the foundation of trust needed to get the DNC to back him.
Two things can be true. He was both not popular enough and didn’t lay the foundation to succeed within the DNC.
Even three things can be true. The two above and Hillary was also unpopular outside of the Democrat party. But she was still popular enough to win the primary.