Portrait on TikTok makes sense because the videos shown in algorithm driven order and are usually around 1 minute long at most. People are also swiping to skip to the next video, and you can't swipe comfortably in landscape.

A 10 minute Quibi episode is already half the length of a standard sitcom with ads (20 mins). It's not meant to be skippable. There really wasn't a need to make it portrait mode.