Maybe the origins and evolution of baseball in the 1800s contain something to explain this but I don't know of any.
Did it have to do with the old fair foul rule? If so, you'd think that fielders would be allowed to play in foul territory.
Was it because they didn't want another fielder backing up the catcher? That eliminates the spectacle of the pathetic catcher chasing after a ball that goes to the physical backstop and farther away from most fans.
More on why there is all that space behind the catcher in a subsequent post. I have a theory.