It just happens to be a focal length that represents the world with a comfortable amount of perspective, similar to what you'd be normally processing with your eye + brain.
Sure, you can look deep into the distance with your naked eye and see a telephoto perspective on something, or pay attention to your peripheral vision and notice a wide angle effect on your world, but most of the time you're probably scanning and attending to the world from a roughly normal perspective.
Just happens that a film diagonal focal length roughly matches that, I guess.
BTW reading over the first bit of the thread I guess my first response was a bit repetitive. Sorry for that.