Extension modules can be, and are, written in Rust and C++. And PyPy has a compatibility layer to run extensions (such as numpy) that are written for CPython.
The reason extension modules are typically in C is of course the API is in C, but that’s true of cffi as well (though you’re right that cffi is more portable). And the reason the API is in C is more fundamental than “CPython is written in C”.
Extension modules can be, and are, written in Rust and C++. And PyPy has a compatibility layer to run extensions (such as numpy) that are written for CPython.
The reason extension modules are typically in C is of course the API is in C, but that’s true of
cffias well (though you’re right thatcffiis more portable). And the reason the API is in C is more fundamental than “CPython is written in C”.