It's "just the way it's done". The first aircraft were set up that way, it worked, so nobody "fixed" it. Think about it. though. Would it feel more natural to push the stick forward to go up? You have to sort of lean forward if you were to push it to the limit of its travel (full size a/c), which is the opposite reaction that centrifugal force is going to apply, same with the converse, pulling the stick back to dive. Actually, regardless of the direction you take, the stick motion sort of "positions" you for the inertial or centrifugal reaction of the plane. Maybe Wilbur or Orville tried it the other ways and puked or something, said "Nah, let's try something else,this way don't work too good", and what we have now is what we have now.