The story of Cain and Abel is also very brief and lacking in details. From their birth to Abels' death is only eight verses in Genesis.
The story does not give any clue as to why God rejected Cain's offering. It seems as if the story is more about Cain's reaction to the result. It sounds more like a cautionary tale about jealousy. I'm not sure if there is any meaning to the way the story assumes that the world was already well-populated with at least one established city. It would indicate that Cain and Abel had lived quite a long time before they offered up these sacrifices and before Abel was murdered. Cain only took a wife after this happened. It has all the hallmarks of a fable.