One problem may be that Sanders has no heir apparent.

[gulliver]

Bernie's movement is not Democratic Socialism, it's "the Bernie movement." It's centered on him personally. When he drops out of the primary (or loses), his movement currently has no place to go.

Who takes over the dream for 2028 when Sanders drops out or loses? If his followers had a fairly solid idea of who that is (AOC?), they and Sanders himself might be more likely to give up. They've lost this primary, but maybe they want another match down the road. If they don't have at least the possibility of one, they might be less willing to leave the current one.