Maybe because Obi-Wan and Luke were both on the Death Star. All of a sudden theres this kid shouting "no" from over the hanger. Obi-Wan dies, they escape, and next thing you know, this loud mouthed kid has blown up your "Death Star"(kills me in Austin Powers). Put 2 and 2 together, and whadya get. Luke Skywalker. Ships that blast off from Tatooine on your Death Star. Obi-Wan on your Death Star. It's a small Galaxy all of a sudden remember!