Lisp was ideal for reasoning systems, its homoiconic and meta-programmable nature is perfect for manipulating symbolic structures and logic. But when AI shifted toward numerical learning with neural networks, tensors, and GPU computation, Lisp’s strengths mattered less, and Python became the new glue for C/CUDA libraries like NumPy, PyTorch and TensorFlow.
Still, nothing prevents Lisp from coming back. It would actually fit modern deep learning well if a "LispTorch" with a CUDA FFI existed. We would have macros for dynamic graph generation, functional composition of layers, symbolic inspection, interactive REPL exploration, automatic model rewriting etc.
We almost had it once: Yann LeCun’s SN (the first CNN) was built on a C core with a Lisp interpreter on top to define, develop and inspect the network. It eventually evolved into Lush, essentially "Lisp for neural networks", which in turn inspired Torch and later PyTorch.
https://x.com/ylecun/status/1944504502260003296?lang=en
So Lisp didn't die in AI, it's just waiting for the right people to realize its potential for modern neural networks and bring it back. Jank in particular will probably be a good contender for a LispTorch.
I don’t think "doesn’t work for teams of 5+" is a fair generalization. There are production Clojure and Emacs (Lisp) codebases with far more contributors than that.
Language adoption is driven less by inherent team-size limits and more by social and practical factors. Some students probably don't like Lisp because most people naturally think in imperative/procedural terms. SCIP was doing a great job teaching functional and symbolic approaches, I wish they hadn't shifted their courses to Python, since that increases the gravitational pull toward cognitive standardization.
So modern AI is all mostly C or even Fortran, often driven from something more pedestrian, like Python.
Lisp languages are great for these manipulations, since the AST being manipulated is the same data structure (a list) as everything else. In other words, genetic programming can lean into Lisp's "code is data" paradigm.
As others mentioned, today everything is based on neural networks, so people aren't learning these other techniques.
In fact, the first edition of AIMA even had a NN and Perceptron implementation in Common Lisp. (https://github.com/aimacode/aima-lisp/blob/master/learning/a...)