User blog comment:Piyush Banerjee/Shin DxD 2 discussion/@comment-34061653-20190104122109/@comment-36523287-20190104130112

Most Issei's friend are noted to be very talented and have been being trained since the young age (10 years or more). Considering that Issei has advanced from a below-average-strength former human  to be on par with the most powerful beings in just around 1 year, His growth rate is already ridiculous enough.

In the term of a plot, Kiba and Rias aren't a worthy adversary for Issei's dormant potentials. Since current Issei is already enough for challenging most warrior-type gods, next powerup is likely to be insane powerful. It's logical to let Issei stay with just Psuedo DxD for a while.