Probably for the same reason that most new language these days cannot bring themselves to just use "function" and instead have "fn", "fun", "func", etc. It's a headlong pursuit of conciseness for the sake of conciseness.

IIRC it was just a personal preference of Graydon's back then. I'm also not sure it was intended to live forever, just something to try and nudge things in a particular direction.

Terseness doesn't inherently mean less readable.

I do think that that rule is probably not one that would be good permanently.