The Warriors don't have to play in Melbourne on ANZAC Day, but they choose to, this is not something the ARLC or anyone else makes them do, in fact it was the ARLC who decided in 2015 to break up the fixture and get the Warriors to play at Mt Smart.
The match drew 15K, the players hated it, the club wasn't happy and the next year it was back.
The Storm & Warriors had to fight for the concept, when it first started in 2009, it wasn't part of any TV contract and the clubs had to convince the NRL to ask for a special exemption to play the fixture as it wasn't at a contracted timeslot.
The game has become a marquee fixture since which for a code which lack many marquee match ups is important.