Because story-telling. Not all stories reveal the villian in the first chapter of the book. The story was about you waking up as a guardian and coming to terms in the world the way it is. As you go about this you discover that the traveler has been damaged/hurt, you then find out the hive are going to invade Earth from the moon. you goto the moon and find out they are draining light from the traveler. You restore this piece. You then find out about the stranger who asks you to come to Venus. The traveler forces you to come across the Vex and explains that if you want to start to heal the traveler you need to take out the black garden's heart. The entire story arc is about you trying to heal the traveler since it is now dormant. This guy goes into more detail about each part of the story. [url=http://www.bungie.net/en/Forum/Post/71798165/0/0]Destiny Story Explained[/url]