A brief history of 35 mm movie aspect ratios in the US

Ratio Introduced Abandoned Technique
1.33:1 Circa 1891 1932 Full frame
1.375:1 1932 1953 Cropped
1.85:1 1953 Present Cropped
2.35:1 1955 1970 Squeezed, cropped
2.39:1 1970 Present Squeezed, cropped

In the late nineteenth century, 1.33:1 (4:3) was first used by William Dickson. This was the standard for all silent films.

Storing sound next to the picture required the image to be narrowed to 1.19:1. People compensated by making the picture shorter too. In 1932, the Academy of Motion Picture Arts and Sciences standardised 1.375:1 (nicknamed the Academy ratio), everso slightly wider than the silent standard. This was the standard between its introduction in 1932 and widescreen's displacement of it in 1953.

In 1953, anamorphic widescreen was introduced. This squeezes in a double-width image, then stretches it back out again to its original wide shape (with a lot of caveats). This allowed everyone to carry on using their existing 4-perforation-per-frame equipment (cameras, projectors, etc) with modified lenses. Again, working out where to fit the soundtrack caused a brief flurry of minor variations. In 1955, it was standardised to 2.35:1, where it remained until a minor revision in 1970.

Also in 1953, non-anamorphic films were cropped wider, at 1.85:1.

In 1970, the anamorphic widescreen standard was revised to 2.39:1, cropping out the very top and bottom of the image to hide artifacts caused by splices.

In short, the space you have to work with is 4:3; you can fit exactly double the horizontal resolution into a cramped space with anamorphic lenses, or you can not do that; and everything else is achieved by cropping, chiefly to make room for a soundtrack. Digital setups make all this obsolete.