Granny didn't finish her story about the man on the bridge because both Terry and Tyrone failed to understand the significance of her tale.
On the surface, Granny's story is about a man who was on the verge of committing suicide. She tells the children that there was a crowd of spectators on the bridge at the time. Among the crowd was the police, the man's significant other, and the minister. She relates that both the minister and the woman tried to talk the man out of jumping. Meanwhile, a stranger emerged from the crowd and started taking lots of pictures of the suicidal man, his woman, and the minister. Granny says that the stranger almost used up a whole roll of film in the process.
By this time, all Terry and Tyrone want to know is whether the poor man jumped to his death. Granny's response is telling:
And Granny just stared at the twins till their faces swallow up the eager and they don’t even care any more about the man jumpin. Then she goes back onto the porch and lets the screen door go for itself.
Granny's main point of telling the story seems to be to voice her disgust with those who exploit another human being's suffering for their own gain. Such individuals exhibit no discretion in their actions because of their inconsideration towards their fellow man.
As such, the stranger in Granny's story has his equivalent in the two men who are taking pictures for the county food stamp program. The men show little consideration for private property and the rights of their subjects not to be photographed against their will.
So, Granny stops telling the story when the twins seem to be more interested in whether the man jumped to his death than the underlying message of the story: true concern for our fellow men always exhibits itself in discretion and consideration. In this respect, neither the stranger in Granny's story nor the two men from the county manifest any true solicitude towards the subjects of their film project.