Originally Posted by
1Aturnleft
... is it because the lounge has capacity issues thus not guaranteeing elites access or is it because it is closed most of the day?
Not necessarily (though the Golden lounge is not that large), it is also possible that QR does not want to spend what the MH lounge would be asking to host its FFP members... which QR decided not to do in their own allocated lounge space.
I believe QR tries to do this in other airports as well, try to shuffle their Business Class / FFP members off to non-OW lounges, even when a decent one exists.