It is generally received wisdom that a large part of Trump's success in the Republican primaries was a result of a collection action problem from there being too large of a field. Each individual candidate realized that whomever moved against Trump first was likely to get burned badly. Each of the candidates believed that if someone else moved first, they would be the ones to step in to the gap caused by a Trump flameout destroying his attacker.
I wonder if a similar dynamic didn't play out on the Democratic Party side, but in the pre-primary stage. Hillary Clinton has long been known to be an especially vindictive player of intra-party politics. Her influence with the Clinton Foundation, the DNC, and the State Department reinforced this. Many of her weaknesses as a candidate were well understood two or more years ago. Yet almost no important player in the Democratic Party even vaguely contested her. That was left to Sanders, someone who had been well outside the margins of Democratic Party power by virtue of being an independent for most of his career.
I suspect a similar situation gripped both parties. Being the first to move against Clinton--especially early in season where she would have years to punish you even before 2016 was too dangerous. Unlikely many unsuccessful primary contestants (say Bush I v Reagan), you could expect seriously going against Clinton to be an extinction event if she won. It didn't seem worth being the first to move.
Either that, or the Democratic Party has a horribly weak bench, such that Sanders really was the best alternative. That seems almost worse to contemplate.