I found the explanation given below here (my emphasis). It may not be authoritative, but it makes sense to me.
You can determine the approximate shape of your visual field by looking straight ahead, and holding a hand out to each side and noticing where you can no longer see them. Do the same for above and below, and you'll see that your peripheral vision probably extends to about 180 degrees (or a little less) left and right, and about 100 degrees up and down. Your brow, nose, and cheeks narrow the vertical angle.
So your visual field has a ratio of about 1 to 1.8.
Interestingly, and not entirely coincidentally, ordinary films and wide-screen TVs have a ratio of 1.78 (16:9).
Another way to see this effect is to look at someone's head in profile and from directly above. In the former case you have a clear view of their eyes, in the latter you don't. (Tested on a small sample - one 3 year old male, playing with an iPad).