Std requires I/O multiplexing which is very complicated with
stdio, because it provides no transparent buffering.

Proprietary UNIX variants like AIX, HP-UX or SCO UNIX
are nowadays rarely used and maintaining compatibility to
non-standard operating systems interfaces is wasteful and
practically useless.

Projects like xterm suffer from backward compatibility to decades-old
UNIX variants and terminal standards. This does not conform to st's
design goals.