A big part is that Python uses reference counting GC. Java, Go, C# all use tracing GC. Py_INCREF and Py_DECREF are responsible for inc/decreasing the reference count, and are not atomic. The GIL ensures refcount safety by allowing only one thread access to changing refcount. The naive approach to parallelization would require locking each ref inc/dec. There are some more sophisticated approaches (thanks to work by Sam Gross et al) that avoid a mutex hit for every inc/dec.

Tracing GC does not run into this problem. Why Python doesn't use tracing GC is not something I am qualified to answer.

Sam Gross' work: https://docs.google.com/document/d/18CXhDb1ygxg-YXNBJNzfzZsD...

The GIL code: https://github.com/python/cpython/blob/main/Python/ceval_gil...

Py_INCREF: https://github.com/python/cpython/blob/a4460f2eb8b9db46a9bce...

python made it's C api visible, so things like reference counting are widely observed by C libraries that interop with python. This makes it much harder to make changes since you can't change the implementation in ways that programs rely on.