Note that the “Authority” in PyPA does have real authority behind it: there is a standing delegation from the elected Python Steering Council ceding control of certain kinds of Python Enhancement Proposals to specific individuals that identify themselves as members of the PyPA. (And before that the standing delegation was from Guido as BDFL, ever since we first set the arrangement up back in 2013 or so)
The difference between PyPA’s design perspective and conda is that PyPA is mostly platform neutral - we definitely point folks towards conda when we think it would help them (see https://packaging.python.org/overview/#depending-on-a-separate-software-distribution-ecosystem for example), but we’re not going to start telling people to stop using non-conda Python distributions (and any of the other distribution techniques mentioned on that overview page).
So even though it’s a freely available open source platform, saying “We only support conda users” would be akin to saying “We only support Enthought Canopy users” or “We only support ActivePython”.
We’ve just been successful enough in our cross-platform design goals that folks initially publishing to one particular platform don’t get asked “Please support (some other particular platform)”, they get asked “Please support the default Python ecosystem tooling”.