Another thing to mention is that until very recently (Python 3.12, I think?) every interpreter in the address space shared a lot of global state, including most importantly the GIL. For my area (audio plugins) that made Python a non-starter for embedding, while Lua works great.

I agree though: biggest reason is probably the C API. Lua's is so simple to embed and to integrate with your code-base compared to Python. The language is also optimized for "quick compiling", and it's also very lightweight.

These days, however, one might argue that you gain so much from embedding either Python or JavaScript, it might be worth the extra pain on the C/C++ side.