The way I see it, st is trying to replace xterm.

xterm is a de facto standard, in that you can sit down in front of a machine which appears to be running some flavour of Unix, it doesn't matter if it's Gnome, KDE, TWM, CDE, etc., GNU or BSD or Solaris or whatever, you'll probably be able to find xterm and work out the rest from there.

Many devs/hackers use xterm as their day-to-day terminal, and seem to approximate ANSI control code compliance with "works in xterm".

The problem with this situation, as mentioned on the st page, is that xterm is unmaintained and unmaintainable.

st is trying to be a maintainable replacement, making improvements like vector fonts, and not being shy about ignoring legacy/niche/solved-elsewhere features.