I think it is the hardware we use to view pictures that is the cause behind this. We all (or most of us) will have a monitor that is longer than it is tall. Mine is 2560x1440 pixels. When I create a picture in landscape orientation that i want to view in full screen I resize to 2160x1440. if I create one in portrait orientation I have to use 1440x1152. It is by its nature a smaller image.
When we only had prints to view our photos they would (usually) all be printed the same size (eg 10x8) irrespective of orientation. Our landscapes would be 10" wide and our portraits would be 10" tall.
I think it is the size restriction that prevents me from using portrait orientation, unless the picture is well, a portrait