User blog comment:B214/Question and Answers 2/@comment-187.209.0.247-20140225185340/@comment-187.209.0.247-20140225220346

But the problem for issei is that under the perspective of all the gods, leaders and politicans devils, angels, etc,,,issei is the most dangerous entity ever created, someone who can not be killed, Indra and Rizevim Livan Lucifer put it in clear, as how the other leaders and Gods watch issei.

And even now it seems that the author is pointing that factor, WORDS FROM VOL 16;

Sensei then says it to Dulio.

'''“It will be bad appearance-wise for a Devil or a Fallen Angel to become the leader. From any perspective, you have an image of them being the villains. For that reason an Angel will have a good image and that will be good for us. There’s no loss for you to take the role. The biggest point will be that you are an Angel that reincarnated from a human. You will have a good image from the human’s points of view as well.”'''

'''Well, that’s certainly true. If we have to choose a leader among these people, then an Angel certainly has a good image.'''

The factor is how will show issei to the world that he never will be a villian, even if is the son of the apocalypse and the infinite, even if he is a demon, maybe if CaoCao betray DxD team. and the True Longinus reject him again, chosen issei as his user, could show that he is a Hero for all the factions that desire peace.