I'd never thought about why telnet uses ^] as its default escape sequence, but I just formed a theory while looking at an ASCII chart.
The ASCII ESC character is 1B, or ^[ (which many vi users know if they have a keyboard without Esc, or prefer to use ^[ for ergonomic reasons).
Presumably telnet has an important requirement that you be able to use vi -- or other software where you'll need to send ESC -- normally on the remote machine. If you're on a system where you're sending ^[ for ESC, what's the natural similar-but-different alternative for a meta-escape? Presumably ^], which is right next to it on your keyboard and conceptually related. That's the GS ("group separator") character, which isn't commonly used for anything on Unix.
The ASCII ESC character is 1B, or ^[ (which many vi users know if they have a keyboard without Esc, or prefer to use ^[ for ergonomic reasons).
Presumably telnet has an important requirement that you be able to use vi -- or other software where you'll need to send ESC -- normally on the remote machine. If you're on a system where you're sending ^[ for ESC, what's the natural similar-but-different alternative for a meta-escape? Presumably ^], which is right next to it on your keyboard and conceptually related. That's the GS ("group separator") character, which isn't commonly used for anything on Unix.