One possible answer is that AI is a collection of very hard problems, and Lisp is a good language for solving hard problems, not just AI.
As to why that is: macros, generic functions, and rich introspection allow for concise code and easy introduction of domain abstractions — it's a language that you can make more powerful. For a lot of problems that's unnecessary, and it comes with its own costs, but for other problems that power is needed to make any headway.
