Why?

Just for fun and to be different

Perhaps my idea of fun is abnormal but for me a good TV set is a considerable expense and to deprive myself of the broadcast material by cutting off large portions of the picture to have a different geometrical shape of the picture does not amuse me. You can, of course, to delight yourself, make a cardboard cutout mask to place before a standard display and have all the fun in the world at small expense.

In the very early days of TV, the CRT was round, although produced a squarish picture. From that, everything was made to be backwards compatible with previous iterations. Also, sending a triangular picture would be technologically hard.

from the standpoint of interior design this sounds great, but to impliment it would be very impractical, the shape of the display must be standardized, otherwise, the picture transmitted would either not fill the shape of your screen, or be trimmed to fit, as seen with the editing of movies, where you miss parts of the view, for movies, the editing actually does not trim a standard part of the screen, it is constantly adjusted to see the important parts, this would be impractical to do with every show, in multiple formats for different shapes