1. The Clinton stranglehold (first Bill then Hilary) on the DNC was absolute to the point with Hilary off the stage there's a vacuum and no real leadership contenders. That I think was largely by design. Hilary didn't want a repeat of the 2008 nomination;
2. Obama seemed to either not care of was ineffective in shaping the party. After eight years in office it was still Clinton's party;
3. The Senate minority leader Chuck Schumer (D-NY) is really the Democratic senator from Wall Street and is one of the biggest defenders of the carried interest tax credit for hedge fund managers.
4. The Democrats have an ageing and increasingly out-of-touch ruling class. As just one example, Diane Feinstein has announced plans to seek reelection... at age 85. This means she'd be 91 at the end of her term.
5. I'm still ot sure the Democrats fully appreciate the anti-establishment wave that swept Trump into office and almost swept Sanders into the Democratic nomination.