Warren has two obvious problems with party elites. First, there is the perception among some of them that her left-wing stands, such as Medicare for All, are too risky for the general election and decrease the party’s chances of defeating President Trump. For example, House Speaker Nancy Pelosi has not outright endorsed Biden or specifically declared that she does not support Warren, but Pelosi has argued that the party needs to have a big, sweeping electoral victory in 2020, and that such a win requires more moderate policies, like focusing on improving Obamacare instead of pursuing Medicare for All. Those are sentiments decidedly on the side of Biden and Mayor of South Bend, Indiana, Pete Buttigieg and against Warren and Sanders.
Secondly, electoral considerations aside, there is a center-left wing of the Democratic Party that fundamentally disagrees with Warren’s more leftward positions. It’s hard to imagine some of these figures endorsing Warren before she has effectively already won the nomination. (That fits with Shor’s findings — Warren’s endorsers at the state legislative level are more liberal than the endorsers of any of the other candidates.)
These problems are not unique to Warren. Sanders was perceived as too far to the left by many Democratic elites in 2016; he got very few endorsements back then and is not getting many this cycle, either. (Sen. Amy Klobuchar actually leads Sanders in endorsement points.)