Are these few that can't get the manufacturer service bulletin because they left their run late or because the manufacturer has been dragging their feet?
It's because those manufacturers sell the vast majority of their aircraft to customers in the USA, Europe, and other places where ADS-B mandates are either a few years away, or there is currently no mandate at all.
The only reason Australia is rolling in ADS-B before the rest of the world, and therefore before the market is ready for it is to give senior CASA staff something to brag about to their mates in Montreal.