Why has Hemingway provided so little information about the past lives and circumstances of the characters?
You'll probably get a variety of answers for this one. This simplest answer is usually the best, though. Hemingway tends to create characters who represent a type--generally, a man trying to maintain masculine control of his surroundings. In some ways Hemingway wants him to be an "everyman." If we know too many details or can explain too much of his past, then we won't be able to relate to him as well. Another likely possibility is that it just doesn't matter (at least the specifics, anyway)--Hemingway wants to focus on the character's present and he deals with problems in the here and now.
The author was very vague in his description of the couple because he just wants them to be representative of a man and a woman. Hemingway wanted the reader to speculate on their past by giving only vague details of the present. It is their present situation that he wants the reader to focus on so that the reader has to be able to read between the lies to determine what is going on in the story as the communication unveils the facts. The dialogue of the story is the most important aspect of the story and Hemingway does little to deter from it with the exception of creating a verbal illustration of the hills of Spain.