Why didn't Ben train Nagato? Because he is old age. The body can't move. Can only rely on. Black and white will never find out about the outside world. At this time, Tu was just injured. Half the body is broken. Put in front of the class. This is also the case. You can't move like this. You can brainwash the soil in class. Although he didn't agree at first. But I saw it in the soil. Kakashi killed Lynn by mistake. This made her completely black. I also accepted Ben's advice. He decided to create a world with Lin. It is used directly by the class.
Secondly. Detu belongs to the Uchibo family and already owns sharingan. He wants to pass. The pupil of the eye rotates to wake itself up. You must transplant your eyes into your body with mud. Obviously, this is more difficult to achieve. So he thought of a medium. It's from Nagato. Find these eyes. Let Detu reuse Nagato to revive Diandian. They are all members of the Neibo family. People in their own families are better informed. After all, Uchihiro Madara is the ancestor of the Uchihiro family. In terms of ambition, he knows the Neiguang family better than anyone else. It can be used better.
Another factor is Nagato, who got a second chance. Has been accepted as a disciple by Jiraiya. Jiraiya will guide him to the right path. He can only let go of some opportunities. Let Nagato lose consciousness to achieve his goal.