There was a long legacy of using 80x25 and lots of software that would assume this layout. I think that comes from an even older legacy of dumb terminals such as the VT100 that used 80x24 plus one line for a status line. BTW 80 columns comes from an even older legacy of IBM punch cards having 80 columns. Basically, anything that wasn't 80x25 was going to break a lot of things

And the 80 columns come (losely) from a still older standard in typography, of about 70 characters per line, found empirically as a good size for a line. Even today is good design practice in UI, Web and books to stick to 60 to 80 CPL.