The technical issue is that the limited C API is badly tested by Python itself. In the past, we introduced bugs which were only discovered too late, after a release. Testing means building a whole C extension with the limited C API, but also execute the code by running tests on it. Currently, the xxlimited module is tested but it’s quite artificial and only covers a small part of the API.
Moreover, there are some functions which are missing in the limited C API which is non-obvious when you only consider trivial C extensions. Converting some stdlib extensions help to discover missing functions, that’s how I come up with proposing to add PySys_Audit(), PyMem_RawMalloc() and PyInterpreterState_IsMain() functions.