User blog comment:B214/Question and Answers 6/@comment-180.253.126.123-20160718170926/@comment-9595671-20160718201551

I think it's partially because from what I see on the manga page, the release dates between each Manga volume are very large. Volume 8 for example only reaches the final events of the the anime's second season.

And admittedly, most manga I've seen have their translations appear based on how popular, so that could play a large role in how often it's updated. I'm not saying the DxD manga is unpopular, but one's more likely to see an update from a shonen manga than from any other mangas.