One reason is because people can't put things on the top … eg. pens, pencils, erasers, books, cold drinks (with water condensed and coming out the base of the glass), hats, pets, etc.
Another could be too tall … and taller than ceiling height. And uprights make it hard for some people on the other side from seeing the piano player.
I'm an upright acoustic fan though. I had a knight brand acoustic upright for ages, until the soundboard started getting issues, as in tuning/re-tubing became a waste of time etc when the board issue arose. Excellent piano in my view and ears. Lasted for ages.