Because you've just invented that. No such thing has ever happened. In fact, one of the points that was recently agreed to by the clubs was designed to do the exact opposite to what you're suggesting.
I didn't conveniently forget anything. A Queen's Birthday game in the late 1990s between Collingwood and Melbourne drew 40k, just. Two years later, it became an annual event. It drew 50.8k last year. The point is that Collingwood against any number of teams (which, pretty obviously, was what I was getting at by mentioning Richmond and Carlton, as alternatives to Melbourne) would get 20k+ more than that, easily.
So, what you're saying is two big Victorian clubs don't need a specific public holiday to for one of its games to be a blockbuster; they just need a prime time spot generally? I think we might be going around in circles then.
But also in 2006 when Melbourne were 4th and Collingwood 6th 78,000 fans turned out!
But how long do we have to wait for the dees to start winning? Maybe another 3-4 years.
Also in 2007 76,000 turned out and in 2010 the drawn game 70,000 fans.
To queens birthday you will get your minimum 40,000 Collingwood fans but it all depends on melboure are going! This year they will probably get 15-20,000 Melbourne fans to the game. If and when they start winning you will get 25-30,000 dee fans along with 45-50,000 pie fans and then you will have a genuine blockbuster