Vertical framing is not a rotated version of horizontal composition. The change in proportion alters what fits, and consequently which subjects the format handles well.
The frame favours one subject
A tall narrow frame accommodates a standing figure comfortably and struggles with anything requiring width, which is why single-person material dominates the format.
Two people side by side must either be pushed toward the edges or shown at a distance that reduces both to small figures within a great deal of empty space.
Placing them at different depths instead, one nearer the camera and one further back, resolves this by using the frame's height rather than fighting its width.
Context has to be shown separately
Horizontal framing lets a subject and their surroundings occupy the same shot, which establishes where something is happening without any additional material.
Vertical framing generally cannot, so establishing context requires either a separate shot or a camera move, both of which consume time the format has very little of.
Many short-form pieces simply omit context, which works when the subject is self-explanatory and fails when the setting is part of what is being communicated.
Movement across the frame is compressed
Lateral motion exits a narrow frame quickly, so a subject walking past the camera is visible for a fraction of the time the same movement would occupy horizontally.
Vertical movement behaves the opposite way, which is why the format handles rising, falling and depth-based action considerably better than it handles travel.
Cameras therefore tend to follow subjects rather than letting them pass through, which changes the operating style as much as the composition.
Interface elements occupy real space
Captions, account details and controls overlay the frame at the bottom and along one edge, and anything placed there is obscured on the platform even though it appears fine in an editor.
Composing with those regions treated as unavailable is the practical response, which effectively reduces the usable frame to a smaller central area.
Reframing existing footage rarely works
Cropping a horizontal shot discards most of its width, and the remaining strip frequently excludes what the shot was composed around.
Material intended for both orientations has to be shot with both in mind, usually by framing loosely and accepting that neither version is composed as well as a dedicated one.