Why did Batman choose to leave his chin mouth and jaw unmasked?

Personally I think that Bob Kane felt that the character of Batman had to have an image of a bat and a man to scare evil doers. Maybe back in ol' days of comic strips a hero with a cover face showing no facial expression was consider too bandish. Who really knows?

Batman was inspired by Zorro I believe - I think it was to give that kind of look. A dude in a mask. We can see a bit of his face but it ain't enough.