User blog comment:B214/Shin DxD 1 discussion/@comment-95.210.189.106-20180727143952/@comment-28884330-20180727225613

The point of making him lose before the final round is to not wank him. A story with an overpowered character who only wins is boring, predictable and unrelatable, nothing more than vapid power fantasy. And also a loss allows for much more character growth than simply winning, especially when Issei has had so much victories and power-ups recently to show him that there can always be someone better in any regard, or many. If you think that Issei would grow as a King by winning all games then you're not thinking critically.

Also, your mention of Issei's team having new characters as somehow a reason for him to not lose before the finals does not even make sense. Many of the teams in the tournament have new characters, so that is completely irrelevant.