It may be completely meaningless, but this is what was told to me 35+ years ago...

The "normal" lens is selected for any camera to offer a 'normal' view of a scene. This 'normal' view would be considered as looking through the camera as it was a plain window. No magnifaction, no reduction.

So, if you look at a scene with your eyes, then through the camera, it looks the same. No magnification and no reduction. The camera just become a window...

Whether or not this is true (seems logical to me), or if the diagonal of the film vs. the focal happened as an optical coincidence, I have no idea... But it's an explanation that stuck with me for a long time...