Why was the Democratic establishment so united behind Clinton in the beginning?

[Averroës Nix]

I don't think that it's fair to hold the Clintons accountable for the poor state of the Democratic Party as of 2016-2017. That is a more recent failing. The Democratic Party did not have the kind of wide-ranging organizational problems that now appear uniquely severe until within the past decade, and when the Clintons were in elected office they were extremely diligent about supporting Democrats at the state and county level in whatever ways they could, from personal appearances, to organizational support, to fundraising.