Less technical and more political. Python has a bureaucracy that can take let’s say “a while” to decide things. If GitHub approached them about this it wouldn’t surprise me if they’re still debating whether to condone it.
There's nothing required from PSF to create own repo. I created one myself on S3, all you need is then just update pip.conf to use it. Pip also has functionality to support primary repo and a backup, so they don't need to make their repo a pass through to PyPI, pip basically can be configured to look up first the custom repo and if package is not found then fallback to PyPI.
