Implementing a "simple virtual machine" for a particular task is greenspunning[1]. Introducing a JIT library is work, complexity, debug and portability issues. LLVM compiles much slower to native code as the Clasp[2] guys notice comparing to SBCL. And in the end, any of these would be at most "competitive" to the simple Common Lisp implementation in speed (which is even portable across the different implementations).

[1] https://en.wikipedia.org/wiki/Greenspun%27s_tenth_rule

[2] https://github.com/clasp-developers/clasp