1. It is ANSI standardized and mature.

2. Common Lisp programs are safe. Some implementations violate safety for performance reasons, but this one certainly won't.

3. Common Lisp has the best error handling mechanism that I know of (Conditions, handlers and restarts).

4. The language is programmable, both at the syntax level (Macros) and at the object level (CLOS with the metaobject protocol). So new ideas from other programming languages can often just be ported.

5. The language has been designed for interactivity and safe incremental redefinition.

6. Lisp has already been used very successfully for operating system development (https://en.wikipedia.org/wiki/Lisp_machine).

This makes it pretty much the ideal choice for such a project.