Wan in  order to spur Luke on to going into Jedi training and defeat the  Empire. In any event, when it came time to make "Empire Strikes  Back", in which Luke begins his training, Lucas drew from the "ugly  creature with mystical powers" mythological archetype (as he did  when creating Star Wars) and created Yoda as Lukes new Jedi master.  Eventually, Alec Guinness (qv) was lured back into the role of  Obi