Problem is, only a few people know about MC's truth (mainly Fire girl and few of her guild mates and the Swordswoman Teacher, don't remember whether Lightning girl know it or not).
Those people from the Guild don't know about it, so why would they allow him first to be the one to question that old Demon?
It seems like they would only allow him later, because he was one of the person who got attacked and helped apprehend him, and would likely be treated as a pseudo S-Rank (MC isn't S Rank yet, because he has to study some more about non-combat stuff).