User blog comment:B214/Volume 22 spoilers/@comment-50.139.232.230-20160610182436/@comment-24.113.252.213-20160610191815

Also, Issei defeated Rizevim Livan Lucifer (a super devil), and Apophis (a Heavenly Dragon-Class dragon).

Rizevim: While we know he's weaker than Sirzechs and Ajuka (this was stated in the LN), he is definitely at least Mid-God-class (like Loki or someone equal).

Apophis: I think it was stated in the LN that during Trihexa's attack on the Norse World, Apophis wiped out/killed an entire army that had some gods or god-level beings in it.

With these accomplishments on Issei's record, High-Class is the absolute least he deserves. I know that a devil's rank doesn't really say anything about their strength (Issei is stronger than any High-Class devil, yet he was only promoted to Mid-Class), and is more of a reflection of their public reputation in the Underworld. So maybe the higher ups of the devil society still don't want a young reincarnated devil to get too much status.