In "The Garden Party," why was the main character undeveloped?

Katherine Mansfield leaves the character of Laura Sheridan relatively undeveloped because writers of this period, Modernism, were experimenting with a new, different style. World War I had recently ended, and the war had produced a very destabilizing effect on modern society.  Modernist writers wanted to capture that destabilization in their work, so many were leaving out expositions, character development, and tidy resolutions at the end of the story.  
Modernist writers wanted their readers to become their partners in creating meaning.  What Laura learned about separations between social classes, reality and illusion, and life and death is left unexplained.  Through observing what Laura saw and heard in the wake of her neighbor's death, Mansfield wants readers to draw their own conclusions about these essential conflicts, rather than using Laura to deliver a conclusive message.  Laura's inability to express herself with regard to what she saw at the Scott's house mirrors the inability of modern people to express what they felt about the experience of World War I and the uncertainties of modern life.