Something that I find a little concerning about Racket is that AFAIK there are no alternative implementations. (Clojure and Common Lisp have a couple). Why is that? is it too complex?
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?
Common Lisp has more than a couple of implementations, especially when we take a historic view. However, say, Steel Bank Common Lisp has only one implementation.