User blog comment:B214/Volume 21 info & spoilers/@comment-187.209.22.243-20160217145113/@comment-1359322-20160217162922

That the thing with DxD team, the team consist of those without major position in the various faction in the world. Sirzechs and his team are a major part of the Underworld, Sirzechs is a Maou and his team is his personal group of bodyguard. This applies to other factions, it's not like Odin or Zeus can easily send their top fighters without thinking.

"“Since an incident happened where the territory of the two greatest factions of the Vampires got taken down, each of the factions will be feeling nervous from now on. Each of the mythologies will have at least one insanely strong God in their forces, but they have to worry about how people will view them, so they are not able to go out and take down the terrorists so easily. That’s why we are in need of a counter-terrorist team that consists of strong fighters that can fight Rizevim and his group head on while being able to move to such places right away.”"

- Volume 16

Also the DxD team is more complex than it seem, sure it consist mainly of Devils but they also have Azazel, Dulio (The Joker of Heaven), Sun Wukong and Cao Cao, both whom Sakra's Vanguard.