User blog comment:B214/Shin DxD 1 discussion/@comment-61.6.217.163-20181104114348/@comment-31137559-20181106115928

Well it just seems as the higher-ups want to wait as some of them haven't been devils for all that long like Asia, Xenovia, and Rossweisse. Because their achievements would warrant them all becoming high-class devils but it seems they wanted them to at least go through some proper time-in-service.

But Issei's accomplishments were too great for them to just wait. So he became a high-class devil a little less than a year after being reincarnated.

But it said that the members of the Gremory group(the ones that weren't promoted already) along with Saji and Tsubaki got mid-class promotion recommendations.