He tried convincing himself you were just a normal evil spirit. He honestly doesnt even know if you're evil. Okay, so its just a normal exorcism. But why couldnt he bring himself to exorcise you?

"Come on Mob, our clients are counting on you." Reigen said in the background, waiting for Mob to do his thing.

"I know master, just give me a moment." Mob responded back. He then realized why he didnt want to exorcise you- you were too pretty.