Another thing that many don't realize is a reason, and one that has affected both parties. Choosing the same candaites to run repeat campaigns after they have already failed once before. See the very fact that Hillary lost her 2008 bid, should have been more than enough of a reason to not choose her in the primaries in 2016. As much as I would have preferred Sanders over both Clinton and Trump, if he ran in 2020, I honestly don't think he would win. I do however, strongly believe that if he had been selected by the DNC, that he would have won.