I have often thought that it would be poetic for newly converted positive 6D entities to incarnate for the purpose of being central figures in illuminating truth. It would almost seem to be a requirement to come and express their newly integrated philosophy, or at least highly desirable as it's possible there may be a type of regret in regards to their past that is felt once the switch is made..though I'm not sure about that. I'd imagine them to have influential personalities, a strong sense of self, which help as teachers.
And if wandering is such a risk, it would make sense that a newly positive 6d entity might feel more comfortable with the gamble because of the end reward should they not polarize. Because if they don't and are required to incarnate in 3d for a great cycle, it only means more opportunity to balance and serve. They effectively would be rich in experience having walked both the negative and positive paths.
I'm sure negative wanderers mask as positive figures, so I imagine Ra didn't want to cause confusion.