Why would you leave the names of the main characters out of a short story?

We're doing a study on the short story Butcher Bird by Wallace Stegner in english class. Instead of having names, the main characters are referred to The mother, the father and the boy. Why would you leave these characters nameless?

A short summary of the story, if you want it, but you don't really have to read it if you don't want to:

The story takes place during the great drought in the 1930s in rural saskatchewan. The family (the father, the mother and the boy) meet their new neighbour, Mr Garfield, who is british, doesn't like violence, and encourages the mother to try and grow some trees like he is trying to do, despite the drought. The father on the other hand, relishes in violence and puts everybody down all the time. He mocks mr. garfield after they have left his house, and scoffs at the mother's idea of growing anything.

So yeah, any help would be appreciated.