Most equipment is calibrated for a viewer’s FOV or around 40-45 degree, which would be around 32mm in Super35. This is the angle of view of the so called “prime seat” in most proper movie theaters, as well as the angle used in the THX specification for calibration of the audiovisual experience.

The main reason for widescreen popularity is purely commercial. Theaters needed to differentiate themselves from TV. Widescreen was a good way to up the ante. Filmmakers followed by producing content for widescreen. Wider screens also more naturally lend themselves to wider auditoriums.

Circular has very strong compositional implications and would feel weird to a civilization with roots in visual arts, which are traditionally framed in rectangles. :)

(2.35:1 is legacy, 2.39:1 is the current standard.)

I am not really a fan of anamorphic lenses. I’d rather crop an image shot with spherical lenses to a wider ratio.

That said, Iscorama and Kowa are popular, and SLRMagic recently introduced an 1.33x anamorphot.
But I can advise on skipping 1.3x anamorphic lenses if you want to get a good anamorphic effect. They are hardly worth the hassle. Technically, the 1.3x anamorhic lens will approximate a 2.39:1 end result when shot on a 16×9 sensor and expanded, but the anamorphic effects (or, defects, to be more precise) you get are minimal. And, frankly, these artefacts are the only reason to shoot anamorphic nowadays. On the other hand, anamorphics are a better fit for 4×3 sensors, imo, as you can then use 1.5x and 1.9x lenses without throwing away lots of pixels, but you’d need another camera for that.

One other consideration, which I’ve skipped in the article because it is not artistically relevant, but still worth mentioning, is the fact that the wide format (2.39:1) is often not really well suited for images which are going to end on the web or TV. Most displays are 16×9 or taller (16×10 is very popular, as is 4×3 in Apple’s tablets) and they will have big black bars on the top and bottom when showing the content. In fact, I am writing this on a 5×4 display, and on this display 2.39:1 can look downright laughable instead of “cinematic”. Just something to have in mind before going wide. :)