Why does Blanche’s rape totally destroy her?

At this point in the story Blanche has a less than tenuous grip on reality. Blanche tells Stanley that she received a telegram from Shep Huntleigh, inviting her to take a cruise of the Caribbean on his yacht. Stanley plays along, for now. We get the sense that Blanche isn't just pretending anymore: she has ventured into the crazy zone. The rape just pushes Blanche over the edge. Her fragile emotional and psychological state become completely shattered.