In the interests of "party unity," the DNC voted by acclamation to strip Superdelegates of most of their power. Superdelegates are made up of DNC members and elected Democrats at the national level. 

In 2016, Bernie Sanders complained bitterly about the influence of Superdelegates who voted overwhelmingly for Hillary Clinton at the convention. Superdelegates are not bound to vote for the candidate who won the caucus or primary in the state they respresent.

But the DNC, citing the leftover bitterness of Sanders supporters, now forbid Superdelegates from voting on the first ballot. 

Politico:

Under the new rule, superdelegates – the members of Congress, DNC members and other top officials who made up about 15 percent of delegates that year – will not be allowed to vote on the first ballot at a contested national convention. The change could dramatically re-shape the calculus of future presidential campaigns, rendering candidates’ connections to superdelegates less significant.

“It’s a big victory for the base of the party,” said Jeff Cohen, co-founder of the online activist group RootsAction.org. “Tom Perez realizes that he’d rather lose 10 dead-enders in the DNC than a couple million activists,” he said of the party chairman.

While long a priority of Sanders and his supporters, the effort to reduce superdelegates’ clout was embraced more broadly in recent months by Democratic Party officials desperate to win over young voters skeptical of centralized party power.

Perez described the change as “historic,” and DNC organizers played a video message from former Vermont Gov. Howard Dean in which the former DNC chairman cast the measure it as an urgent response “to the will of grassroots voters.”

Many young voters, Dean said, “have lost faith in our party’s nominating process, and make no mistake, this is a perception that’s cost us at the ballot box.”

The radical left has won a big victory. No Democratic convention since 1948 has gone beyond the first ballot. This gives a genuine shot at the nomination to someone far more radical than an establishment Democrat like Joe Biden. 

Indeed, so called "centrists" are the big losers. Outside of Biden, it doesn't seem likely that a more moderate alternative to the Sanders/Warren wing of the party can emerge to win at the convention. And while it's still two years away, the party is being pulled ever leftward by the emergence of far left activists who are pushing Medicare for All and free college tuition as litmus tests for the next nominee. Even if someone perceived to be more moderate than Sanders/Warren gets the nomination, they will have been driven so far left that they may be unelectable in 2020.

Even with a favorable political climate, Democrats insist on shooting themselves in the foot.