Cisneros uses short sentences and sentence fragments in "Geraldo No Last Name" for a few reasons. First, these sentences replicate the way Marin might have explained what happened to the authorities and to others who asked. She did not really know Geraldo, and she likely had had to repeat the same story again and again. Therefore, she likely used staccato sentences out of exhaustion and because she really did not know much about Geraldo.
In addition, these short sentences leave the reader with the sense that we, and the people in the story, don't know much about Geraldo. His life has been cut short, and the people who were with him when he died did not know any of the details of his life. As Cisneros writes, "They never saw the kitchenettes. They never knew about the two-room flats and sleeping rooms he rented, the weekly money orders sent home, the currency exchange." Geraldo clearly had had a deeper, fuller existence, but he died almost anonymously, without friends or family who know who he was. These short sentences leave us, as readers, with the sense that there is a fuller story about Geraldo that we don't know.