Castro was a shambling monster from another era just as Cuba was a museum piece of Cold War Communism. He belonged in another time and was a contemporary of another war when the left was far less easier to associate with tyranny, when it was less subtle and able to hide its crimes behind euphemisms about social justice.
The left loved Castro for precisely that reason. They worked for Communist Cuba, a goal that they appeared to recently achieve through Obama, because it reminded them of their final objective.
Castro's death has great meaning for them. As long as he was alive, that old Tankie Communism seemed to be thriving in at least some nearby part of the world.