I'd suggest that those that participate in Powerplay but don't enjoy PvP are currently playing it for both.
People play if its good. The people that have stayed in the feature are the ones who see its potential. The people I chat to from my leadership days don't care about modes, they see that as baggage rather than scripture.
One proposal would gate content behind a hostile PvP barrier. Not every player agrees that that is a desirable outcome.
But its not gating it; you are splitting down logical chunks in ways that work. CQC is gated, part of PP would be Open. But you would still have a lot of it available still with missions and the BGS.
It's entirely about gating - that is the whole point of making any game feature Open only. It's about making all participants play in a manner that only some choose at the moment.
Its making parts work the best way they can with FD spending as little time doing it as possible. Thats the heart of the issue- if FD won't put the time in, then this is as much as can be done.
Probably because the lack of participation in the feature made it difficult to prioritise in terms of features competing for resource allocation. Engineers, exploration and mining are available to all players (who own Horizons, in the case of Engineering), in any mode. Open only Powerplay would not be available in all game modes and would therefore cut its likely playerbase before any development began.
You don't know that, just as I can't definitely say otherwise.