It comes from the times when the screen were cathode-ray tubes. Late CRTs, as in old TV screens, were rectangular. But early CRTs were made circular for technological reasons, even if the device used a CRT as a rectangular screen. For a circular CRT, the diagonal of the rectangular "screen" part was just the diameter.
Since CRTs were invented long ago, and the first scanned video signal image was transmitted in 1907, the industry had time to develop a tradition which is not going to go now.
It comes from the times when the screen were cathode-ray tubes. Late CRTs, as in old TV screens, were rectangular. But early CRTs were made circular for technological reasons, even if the device used a CRT as a rectangular screen. For a circular CRT, the diagonal of the rectangular "screen" part was just the diameter.
Since CRTs were invented long ago, and the first scanned video signal image was transmitted in 1907, the industry had time to develop a tradition which is not going to go now.