POSIX was basically a committee ratifying things that a bunch of systems people had come up with. So it gets to the point, it's reasonably concise, and there's little noise to wade through to do most things.
Java was slowly designed by committee, taking inspiration from other languages.
I just thought of something. Maybe it's relevant.
IBM said "let make something totally different with this 8086 system, let's make everything open and not lock it down." Apple said "let's do something different with the Macintosh, let's enforce absolute control over the hardware and software so we can perfectly unify them."
Maybe Sun got fed up with SunOS and said "let's make a language and runtime that's totally different - something so different that it resists influence by platform architecture."