Why didn't anyone ever make a 35mm camera with a hotshoe atop the portrait orientation? Seems like it'd be easy to have one there as well as on the landscape top.
I would guess that it would add clutter and expense and not needed or used by casual snap shooters. Serious users and pros probably would use a handle mount flash or a flash bracket and would hook it up through the sync outlet.
Anything mounted on the camera more than likely would cause Red eye in the photos unless it was bounced off a ceiling.
I think they don't put one there, because it would lead to many flashes being snapped off at the shoe and it would be uncomfortable to hold a camera that has a hot shoe on the side.
And it's another reason why the square is such a nice format.
Cost and necessity. Few requests for it. An alternate exists.
No ready place to put it that wouldn't interfere with handling or jab you in the hand or whatever. With the older flashes, if you touched it while you had a flash attached elsewhere, you could have gotten a nasty zap, unless they spent more money to prevent that.