The two main things you want are the ability to do experimental programming and the ability to do unconventional programming.
When you're doing AI, you by definition don't really know what you're doing. (If you did, it wouldn't be AI, would it?) This means you want a language where you can quickly try things and change them. I haven't found any language I like better than Common Lisp for that, personally.
Similarly, you're doing something not quite conventional. Prolog is already an unconventional language, and Lisp has macros that can transform the language tremendously.
