a lot of left-wing radicals. Bernie is famous for not suffering those he considers fools, not even lightly. He apparently draws the ideological line directly to his left, not just directly to his right.
(Useless fools | BS | Corrupt fools)
While campaigning he tried to draw as many from all points of the ideological spectrum as he could, and he may feel he still needs to hold onto as many as possible for the future. But maybe not too. Or perhaps he'll feel they could always be rallied again when the time comes.
Whatever, I think there's a clue to his thoughts in the fact that he is now endorsing Hillary Clinton.