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

[The Mikado]

It's worth pointing out that the Clintons have been going around campaigning and fundraising for party figures for literally a quarter of a century and had built up quite a library of favors from party figures from top to bottom. The Clintons had been there for a large variety of figures and they ended up there for the Clintons.

This is neither a defense nor a criticism. Politicians who take care of their allies get taken care of in return, and the Clintons had been building up favors since before Bill was even President and never stopped doing so.

[The Mikado]

Remember that one of Clinton's key advantages (especially compared to 2008) was the near-unanimous support among African-Americans, a key part of the Democratic coalition and basically the cornerstone of the party. Black support is the difference between Obama narrowly beating Clinton and Clinton comfortably beating Sanders 8 years later.