Nim requires a runtime to operate. Figuring out how to safely share the runtime between different Python extensions requires the cooperation of all of those extensions. You probably also need to move all nim code into a thread.

Just a guess, but it is probably because they each have their own GCs, and that makes it harder* to ensure that objects created in language A, from language B, haven't been collected when language B needs them again.

* harder, but not impossible