The NRL isn't holding these tickets back for the sake of holding them back. Trust me, the NRL, Ticketek and the venue want to sell everything as quickly as it possibly can.
For the Finals, tickets need to put on hold for NRL sponsors, club sponsors (who get at least the option to buy seats, if they aren't granted them), player & officials allocations, stadium members, other corporates, other third parties (like RL Experiences), promotions, and to handle any f#ck ups along the way.
I know from having to sell events, you get annoying things as well which see's things unintentionally put on hold for longer than wanted such as a stadium seats are noted as being broken and then it's discovered they are actually fixed, or Ticketek loads an old build. Sponsors all the time get to a few days before an event and then tell you they suddenly don't need half their allocation.