My guess would be that he doesn't pick Robin because Robin already IS a hero - he's fighting the good fight, making a name for himself in his own way. It would be a discourtesy to suggest to a superhero that he give up an identity that he's made his own and take up a new one, especially when there are lots of other people who could do just as good a job. Also, Robin is pretty young for a Green Lantern at that point, isn't he?