User blog comment:Faisal Shourov/Why is Highschool DxD so popular?/@comment-1359322-20160324224359

Simple answer: Boobs & Asses.

Okay jokes aside, i'd say it's the protrayal for the characters mostly. Just take the MC for instance, Ise is a pervert and he is open with it, this by itself already made Ise different from most LN harem MC like Ichika, Lux, Ayato or Shido which all seems to come from the same archtype. And then there's the existence on prominent male characters with proper relationships something most harem LN don't have at all.

LN that only have 1 prominent guy.
 * IS - Ichika
 * Bahamut - Lux
 * Asterisk - Ayato
 * DAL - Shido

LN that only the MC gets the girls.
 * Campione!
 * World Break
 * Aesthetica
 * Shinmai

Finally there's the setting. Despite it being called "High School", DxD has a broader focused battle AKA the world, unlike Asterisk or IS which are all school based or World Break or Bahamut where the school students does all the work most of the time. Going into the LN for a bit rather than the anime only. The first three volumes for DxD may have like no adult intervention but after Volume 4, there's always an adult or senior figure helping out in threats.

And so on, the existence of such people are not conveniently forsaken just for the sake of maintaining a "high school" gimmick.
 * V4 - Azazel
 * V5 - Tannin
 * V6 - Sirzechs, Tannin