Building Clojure on the JVM was a genius move by Rich Hickey - it meant that an advanced runtime platform (the JVM) and a huge ready-to-use library ecosystem was already there from day 1.
Backwards compatibility with Common Lisp was never a goal. The JVM ecosystem is bigger by far (and IMHO more valuable as a target).
What about Scheme? Sure, it was bootstrapped in Maclisp, but not too long after had pretty much "zero compatibility and zero code sharing" with mainline LISPs. But is it not a LISP?
1) the Lisps: they carry the name and share basics: Lisp 1.5 ... Common Lisp, ...
2) the derived dialects with some compatibility: Scheme, Interlisp, ...
3) the derived^2 dialects: Logo, MDL, Dylan, Racket, Clojure, ...
4) the derived^3 dialects: ML, ...
The Lisps is category one were and are able to share code. Emacs Lisp implements some of Common Lisp. Something complex like the LOOP macro was once a single source file for Maclisp, Lisp Machine Lisp and Common Lisp.
Clojure shares no source code with any other Lisp. It renames basic vocabulary ('atom') and lacks some older concepts. It's basically a new language.
Everything in Clojure had to be implemented new. There is zero compatibility and zero code sharing to/with Common Lisp, Emacs Lisp, Autolisp, ISLisp, Standard Lisp, Maclisp, Lisp Machine Lisp, ...