The choice of Common Lisp is less flattering than one might hope because it was largely the result of familiarity (Carl de Marcken's doctoral thesis was in computational linguistics and he was most comfortable using Lisp). When I asked him (c. 2007) whether he would have chosen Common Lisp again, he said that he wouldn't have and that he would have chosen Java instead. I don't recall any mention of technical reasons during that exchange (maybe static analysis?), but I do vaguely recall hiring considerations.

(Also, while much of the "business logic" was written in Lisp, a good chunk of low-level stuff was written in C++.)