Well, this is because "normal" programming languages are one step above AST. So LLM has to work with program text, which is much easier than regular human text, as it is constrained to well defined number of keywords and grammar, but still this is pretty variable. Lisp is just AST, so it is one level lower. I guess that at some point LLM-s will stop writing human-readable code, as this is additional obstacle, they will work directly with binaries or virtual machines code (like in Java), because this will be easier and eat less tokens.