I think that the 3:2 ratio has been with us since Leica introduced the 35mm film media. I can't remember the reasons behind it, but I do vaguely remember from classes I took something about closeness to the golden ratio, and how pleasing that was to the eye.
The reason Oskar Barnack/Leica chose 8 perf 35mm film is simple. It's just two 4 perf 35mm Motion Picture Frames https://en.wikipedia.org/wiki/35_mm_film
Nothing magical or mystical.
BTW the REAL Normal Lens for 35mm film is 43mm (diagonal of frame). Leica supplied 50mm lenses, and almost everybody else copied Leica (Pentax made 43mm lenses).
Portrait prints from a Portrait Photographer are 8x10 and 16x20. Mamiya made 6x7 Medium Format cameras because it is about the same ration as 8x10, and therefore required little cropping.
Common picture ratios.
6cmX6cm Medium Format camera = 1:1
6cmX7cm Medium Format camera = 1.17:1
4"X5" camera = 1.2:1
Magazine Page = about 1.29:1
4/3 = 1.33:1
Pre HD TV = 1.33:1
35mm Still Frame = 1.5:1
HDTV = 1.78
Motion Picture = 1.85:1
Wide Screen Motion Picture = 2.35/2.40:1