I’m not sure if the people most-involved with Python packaging (maybe @FRidh @jonringer @adisbladis @DavHau?) agree, but my impression is that it’s because Python package management is largely an imperative mess, and it’s hard to square an imperative mess with Nix.
Because they’re hard to square, people have tried again and again to build good toolchains for automating this. Now there are multiple generations of toolchains for doing so, some of which are no longer maintained, and newer users generally won’t have perspective on where to start.
(This is not novel to the Python ecosystem; there’s been more than one attempt at building a toolchain for most of the language-specific package management ecosystems.)