I don't live in either city, I have family who live in both Melbourne and Sydney so I travel to each city relatively frequently at least a couple of times a year. My experience was Melbourne had their system years before Sydney had Opal cards.
I can distinctly remember first time using tap on/off card to get around Melbourne and thinking how much more convenient it was then needing to use Paper ticket machines Sydney had at the time.
Wikipedia lists 2008 adoption for Melb vs 2012 for Syd that seems to align with my memory. So might have just been curse of them being an early adopter.