User blog comment:B214/Question and Answers 6/@comment-73.57.25.92-20180308230838/@comment-120.188.94.255-20180309203347

Well, it hasnt been stated why ,but it might be because Issei is well known and quite respected by other mythological forces, so yeah he is quite influential.

Zekram actually doesnt mind issei being a maou, what he doesnt like is Ajuka giving maou seats to extra demons like Belphegor and Mammon, thats what i think at least