What if the Island has a soul of its own. Like it accepts the person and rejects the person. It would make sense because look at it this way the world government has the resources to just purely search the new world for Raftel but they haven't. What if that means that Raftel accepts the person who is going to the island. In order for the user to get on they have to have a pure soul. Of course that is just a theory based on the fact that the government hasn't gone yet, but from our knowledge it seems that the only person in the government who could "possibly" read the poneglyphs is Imu or the Goresei. or both. of course the other reason why is that because of the fact that they can't read poneglyphs.