Luke being trained at all was circumstantial. I mean, it's not like Obi-wan went over to Luke and was like, 'time for you to be a jedi'. He recieved the message from Leia, and then decided to train Luke (because he needed help), and Luke let Obi-wan train him because there was nothing else left for him on Tattooine since his aunt and uncle had just been killed.
Obi-wan couldn't have chosen to train Leia because he had passed on before he could even meet her. Also it was better that Leia not be trained by yoda because she was busy with the rebellian, and if vader and the emperor sensed her using th force she would become an even bigger target.