The top two reason (and I can't remember which is most popular) are:
1) Early houses didn't have locks, so a visitor would knock and since there was no reason to go to the door to let them in (or too lazy?), the response would be, "Who's Here?"
2) A lot of fighting took place in the days of the settlers (probably accompanied by grain alcohol). Evidently it was somewhat common, or just very memorable to tear your opponents ear off. When the detached ear was spotted on the ground, someone would yell, "Who's Ear?"
I think I have them in order of most popular belief. Thinking about it, I would almost say the Preacher Hoosier is almost more viable. At that time in history, settlers would have never admitted that their state nickname came from a black man, and would invent any story to change history.
In public school history, we were only told about the first two. In Indiana history in college, was the first I ever heard of the preacher.
Out of curiosity, where does the northern (Canadian) term Hoser (sp?) originate? There is a lot of similarity, and the change in accent might account for the difference in spelling/pronounciation.