Racket itself came from PLT Scheme which was based on the Scheme standard (R5RS or R6RS at the time of Racket's diverging, can't recall which, wasn't paying much attention). It's still a fairly new language (13 years old now) and was largely Scheme compatible most of that time (still is, actually, if you stick to `#lang racket`, but not 100%). It also has no published standard, in this regard it's somewhat like Rust and others where to make an alternative implementation you've got to reverse engineer (fortunately it's open source) aspects of the current implementation.

So it's not like you can just pick up R8RR (since it's on version 8, and keeping with the naming convention of Scheme standards as in R5RS) and start building something.

Common Lisp also has a standard for implementers to follow, and is about 25 years older than Racket. That's given implementers a bit more time.

Regarding Clojure, what's the status/quality of all the other implementations?