MEL is 20km from Melbourne CBD, WSI is 50km from Sydney CBD. SYD at 10kms from CBD will always have a huge advantage over WSI for tourists and most Sydneysiders.
When Tullamarine opened it replaced Essenden so it was always guaranteed usage because there was no alternative. So not really a comparative situation to WSI, where SYD is a real alternative.
Avalon (which is only 5kms further from CBD than WSI) on the otherhand has never really built up a decent user base. Much closer situation to WSI, secondary with fewer passenger services.