Why is Sanders lagging in endorsements?

[Mehmentum]

The main reasons are:

a). He's unelectable, and not just because he's a socialist.  If a far left candidate emerged that was younger and more charismatic, you'd see at least a few people endorsing them, instead of the absolute silence Sanders has been receiving.

b). Clinton is The Frontrunner.  Nobody wants to go up against a juggernaut.

c). The Clintons have a lot of influence in the party.  I don't think its quite as big as the Bushes are in the GOP, but its definitely there.

But there's also:

d). Sanders is technically from outside the party.  He's (purposefully) run as an independent, and that probably turns some in the establishment away from him.

e). Clinton has worked hard to position herself at the center of the party.  Sanders has some unorthodox positions.  Yes, there's his left wing economics, but there's also gun control and immigration, where his views are to the right of most Democrats.