Rainer Joswig sent me a link to Back to the Future: Lisp as a Base for a Statistical Computing System. I haven't read it, but Rainer helpfully summarized it on comp.lang.lisp. Here's part of his summary:

These guys moved away from XLisp to languages like R. Now they feel that R has not a bright future as a implementation of an efficient programming language. Going back to XLisp is no option, since it is no longer used/maintained and also relatively 'slow'. The authors of the paper argue that by using Common Lisp:

they can write more code in Lisp and less code in C. since Common Lisp implementations have a wider range of performance (due to providing optimizing, incremental, native code compilers) and the implementations are mostly written in Common Lisp themselves.

they get the choice of several Common Lisp implementations that are maintained, so they don't have to maintain the language implementation.

they get a standard language with features and extensions that XLisp does not provide or in a more primitive form. They don't have to maintain/invent their own programming language.

Their goal is to get an interactive system and reasonable performance at the same time - something that several Common Lisp implementations can provide.