Clinton’s dominance on this measure is one key reason why she’s in such a strong position for the nomination: she had already locked up substantial support among Democratic Party leaders. Barring some catastrophic event — perhaps involving Clinton’s e-mail server, perhaps something else — it was going to be very tough for Biden to offer a serious challenge.
This is all the more likely because an actual Biden campaign would have exposed him to the scrutiny and criticism he had thus far avoided. I pointed this out a few weeks ago in response to Chuck Todd’s tweet about Biden’s current strength in horse-race polling:
This is a tough year for any Democrat to challenge Clinton — least of all one that cannot offer a reasonably clear ideological contrast, as Sanders can. Although the Republican race remains uncertain and fluid, the Democratic race is so far a case study in The Party Decides. And, as Pat Egan noted a few weeks ago, Biden’s departure likely only makes Clinton stronger.