You're assuming the DNC even wants to win.

They are a corporate machine with a lot of the same donors as Republicans. Their job is to provide a nominal resistance to the oligarchy, not actually win, govern, or fix things. That's why they don't want Bernie who is not on board with their mission. He was so popular his ideas dominated the primaries, so the DNC pulled out all the stops to keep him from winning, because it would've been very hard to maintain their soft obstructionism with him pushing on so many issues from the bully pulpit.

This is part of why I think you actually should vote for the Dem candidate as long as they're not the raging fascist that Trump is - they don't care about your protest vote or whatever. They will just lose, shrug their shoulders and start fund-raising for the next season where they will continue to be the only viable option because of deeply entrenched structural obstacles to any alternative.