My theory's more boring: By the time CRT-based terminals became popular, CRTs were basically square. And many of the characters of the Roman alphabet are typic

My theory's more boring: By the time CRT-based terminals became popular, CRTs were basically square. And many of the characters of the Roman alphabet are typically rendered about twice as high as they are wide. Then add in a half-line or so of line spacing, and you are now at 26 or 27 lines.