Various reasons; In Roman myths, the mentor was always killed as a sort of test. If the pupil could let go and survive on his own, he was a great warrior... I think. I haven't looked it up recently. Another idea is that he could help Luke through the force better than he could in a mortal body. I hope this helped. If you played and KOTOR games or read " Jedi dont kill un arm enemy" ALso kenobi couldnt kill his " brother Jedi try to avoid killing unarmed opponents. Obi-Wan decided to let the Force decide Anakin's fate.