I really have no idea. I think those who watch the final at 5am - 8am on a Monday morning are football fans. I struggle to see (or understand why) that (a) there is a large number of those people who are football fans that would not watch the Socceroos games (inc. games that were on at a decent time on a weekend) and (b) the number of people who fit into the first category is significantly higher than the 'house wife' population that will watch the Socceroos as it is the 'thing to do', or an act of national pride.
It may simply be that being a Monday morning meant that most people watched at home on their own TV and not in gatherings, which made the viewership data appear higher.
I think it's more that 30% of Australian citizens were born overseas. Argentina vs Spain in the world cup final is going to be a much more enticing fixture than Australia v Turkiye to the over 1 million English and Indian Australian residents, for example.