Basically: Historical reasons. The name “PyPA” was a joke by the pip/virtualenv developers and it was only pip and virtualenv so it was on Github.

here's an anecdote.... per the pypa.io history page,  'Other proposed names were “ianb-ng”, “cabal”, “pack” and “Ministry of Installation”'

https://www.pypa.io/en/latest/history/

maybe even funnier that we have a history page, but it's easy to forget all that's happened, so I made one awhile back...