Human vision is "widescreen."
I'm an amateur photographer (cited as background not as expertise), and I remember as a kid the whole world of options that opened up simply by turning the camera 90 degrees. As I got older and started watching people around me taking pictures, it intrigued me how this one simple little trick was lost on so many. When I would recommend a vertical shot (think, family in front of the castle at Walt Disney World), people would be amazed at the picture tha could be obtained.
The design of the cameras did a lot to force this. The 35mm format and cameras were designed for the landscape nature of our vision and our horizontally-oriented hands. Other formats, meant more for studio photography, which often made their way into vertically oriented magazines, weren't tied to this same constraint. Thanks to the design of camera phones, vertical has become much more normal. Unfortunately, that extends to video from these devices as well.
Vertical is fine for still photography. The image isn't changing so it gives our widescreen vision a chance to take everything in. Video and movies, though, are meant to be an extension of our eys in the real world. Even if the movie is shot vertically, the image is presented to eyes that are used to seing horizontally. It forces an uncomfortable translation.