there is also another thing:
50mm is the more discreet or neutral lens. It doesn’t overwhelm with its form. I have the feeling HCB preferred things simple, no contrast exaggeration or dark room manipulations and the choice of a 50mm supports this philosophy
Perhaps it is also due to the perspective a ‘normal’ focal length lens provides the viewer. The lens and the viewers’ eyes are processing the visual data with a common perspective.
Our stereoscopic vision fails and the world is 2D in appearance beyond 60 feet from our eyes, so beyond 60 feet we depend on how big something looks to determine its distance relative to our position (perspective). For example…We can get a good idea how fast a car is driving away from us by how quickly it gets small. But through binoculars something seems a little off…the car is taking forever to get small and the fellow with the 24mm lens on his Nikon lost sight of the car a while back.
So if other than a ‘normal’ lens is used, the relative size of objects in the scene will be different than experienced in our visual reality. Which in turn can be used to influence the viewer.