Applications can overwrite dependency versions they want to use. It's just not possible in the python package set because if ever two versions of the same package are in one environment very strange things happen and everything falls apart.
And Nix users can totally override specific Python library versions as one-offs for their own Python packages or environments.
It's just really unfortunate that some of Python's internals make that unfeasible for Nixpkgs' collection of Python libraries as a whole. :(