At the end of the day, the only ones who can choose to make the subreddit private are the mods, and I don't think they've given any public statements on it either way. It may simply be that the mods aren't unified on whether or not to join.
lemmy.world has been pretty open so far.
the 1-2 day blackout is a shot across the bow. a lot of subs have committed to going private indefinitely on 6/30, I know there's at least some that are planning on that as well, but haven't gone public about it.
Not money per se, but the oil of the 21st century: data.
I guarantee it's primarily about improving their ability to harvest and sell user data.
The spaceship